1. Definitions & Core Concepts
5. Behavioral Medicine models:
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Public health is defined as the organized efforts of society to keep people healthy and prevent injury, illness and premature death. It is a combination of programs, services and policies that protect and promote the health of all Canadians
- Public Health Agency of Canada
Traditionally, the goals of public health were to ensure a safe water supply, safe food and protection against infectious disease. However, most agencies now take a broader perspective: the city of Ottawa Public Health Department, for example, argues that:
"Public health means more to our city than the prevention of infectious disease, illness and injury - it is about creating healthier environments at home, at work and in the community, promoting healthier lifestyles and choices, adopting programs and initiatives that lead to healthier children and families, and offering protection to the health of all citizens.”
The goals of public health have come to include topics such as emergency preparedness, readiness to handle epidemics (such as influenza), accidents such as an air crash, or terrorist attacks. Most of these threats are inordinately hard to manage when they do occur, so the role of public health can be seen as "Avoiding the unmanageable and managing the unavoidable."
The distinction between population health and public health is that population health takes a very broad, ‘high altitude’ perspective on the social determinants of health and proposes that interventions focus on health inequalities in general (rather than on particular diseases or environmental hazards, as with public health). Population health interventions propose changes in policies, whereas public health includes the practices and procedures required to achieve a desired state of population health. Because population health was promoted largely by social scientists, great emphasis was placed on social and behavioral determinants, and less on environmental factors.
A typical list of public health responsibilities includes
- Health promotion
- Health protection
- Disease and Injury Prevention
- Population Health Assessment
- Emergency preparedness
- Vulnerable Populations
Surveillance: collecting, interpreting and communicating health data and acting upon it in order to identify disease trends, emerging pathogens, etc
Health promotion: encouraging healthy behaviors (e.g., smoking legislation), building healthy environments (bicycle paths, etc).
Health protection: more specific than health promotion, this targets particular health hazards and reduces people's exposure to them. Drug safety regulations; food inspections; hazardous substances and pollution; vector control and smoke-free spaces are examples
Disease and Injury prevention: Closely related to health protection, this focuses on particular diseases/conditions. Many approaches are best implemented at the population level: food safety; immunization; outbreak control; road safety; playground design, etc.
Population Health Assessment: monitoring underlying social trends that affect health (poverty, homelessness, drug use, etc.) and encouraging government s to address these. This relates to health promotion, but moves the focus of attention upstream, to address underlying determinants.
Emergency Preparedness: Natural disasters, infectious disease, bioterrorism, etc. An aspect of health protection, but more generic as we do not know precisely what we must protect against
Vulnerable Groups: Protecting the health of particular groups such as maternal and infant health, refugees, etc.
Link: teaching public health in medical schools from the Association of Faculties of Medicine
Chief Public Health Officer's Reports on the State of Public Health in Canada
"Public Health" : What's in a name?
Public health has been known by many names, such as Social Medicine, Social Hygiene, Community Health, Community Medicine, Population Health, Public Health Medicine or Preventive Medicine.
It is unfortunate that practitioners have felt a need to reinvent the discipline so many times, as it implies something of an identity crisis. The ‘Social Medicine’ title was largely a British invention, introduced in the 1940s to emphasize the inclusion of social factors as health determinants. The name was never fashionable in the U.S., perhaps because of its socialistic implication. In the 1960s and 1970s ‘Community Medicine’ became fashionable and was adopted by the Canadian Royal College for its residency training in public health. Then, in Britain, concern arose that community medicine might be somehow confused with family medicine (intolerable!). So ‘Public Health Medicine’ was proposed to emphasize the medical identity. Meanwhile, in the U.S., the term ‘Preventive Medicine’ remained constant, sometimes merged with family medicine. By the mid-1990s, the term ‘Population Health’ became fashionable. The distinction is that population health describes the condition, whereas public health includes the practices, procedures, institutions and disciplines required to improve population health. Time will tell which term comes next.
This is a true Canadian preventive success story: the numbers of collisions, fatalities and injuries has fallen steadily over time. This is true whether the statistics are expressed as an absolute number of fatalities, or as a rate per person, per vehicle, or per million kms driven:
(Source: Transport Canada web site)
In 2005, there were 9.1 road fatalities per 100,000 population for Canada as a whole. The figure for Ontario was 6.3; for Quebec it was 9.3; for B.C., 10.8; for Saskatchewan, 14.8; and for the Yukon, 19.4. See Transport Canada web site.
Ontario now claims to be the safest place to drive in North America (see the Ministry of Transportation's web site):
Falling numbers of fatalities on Ontario roads
and fatality rates per 10,000 drivers, 1970-2001
More Cars but Fewer Collisions per Million Kilometers Travelled,
collisions per million
Vehicle safety: Bucket seats are legal in some countries:
Evidence-based policy-making sounds like a good idea, but social and economic pressures often lead to evidence being systematically ignored.
Lead is a convenient metal with many useful properties; it is easy to extract and can be immensely profitable. By the early years of the twentieth century, lead was being used in paints, in solder for food cans, in joining water pipes, in pesticides and fruit sprays, and even for toothpaste tubes.
However, lead's health effects have long been known. It is a neurotoxin that can irreparably damage the brain and central nervous system. Overexposure can lead to blindness, kidney failure, hearing loss and probably cancer. Around 350 B.C. Hippocrates recognized a link between smelting lead and abdominal colic. In 1767, George Baker linked Devonshire colic to cider that had been contaminated by storage in lead containers. In the 1890s, childhood lead poisoning was linked to lead-based paint by studies in Australia. As early as 1909, France, Belgium and Austria had banned the use of lead-based paints inside houses.
In 1917, Kenneth Blackfan (a pediatrician at Johns Hopkins) summarized the literature on lead poisoning of small children (who tended to cut their teeth by chewing on painted toys or crib bars). In 1922, the League of Nations argued for a worldwide ban on interior paint containing white lead, but in the same year the US Bureau of Standards noted the great durability of lead-based paints and actually recommended their use in private homes and also in schools. Canada did not restrict the use of interior lead-based paints until 1976, when it reduced the allowable content to 0.5% by weight.
And now to the gasoline story... As gasoline engines were being improved in the early twentieth century, it was clear that larger and faster airplanes would only become feasible if the engines had a higher power-to-weight ratio. One way to produce more power is to raise the compression ratio, but this causes "knocking" or premature ignition because of the relatively low octane of natural gasoline. Several additives can enhance the octane rating of gasoline ways to enhance power: in 1907-08 US Navy engineers had shown that alcohol can do this, but at that time there was not yet a clear need to enhance gasoline. In 1921, Thomas Midgley was working at the General Motors Research Corporation in Ohio and discovered that tetraethyl lead could also raise the octane rating, and this would happily avoid any social censure associated with alcohol products in the prohibition era.
In 1923, a consortium of General Motors, Du Pont and Standard Oil registered a patent on tetraethyl lead as an additive to gasoline, leading to an immensely profitable business (see Bill Bryson: "A short history of nearly everything". Anchor Canada, 2004, pp 149-152). Soon plant workers were showing signs of organic lead poisoning, which management denied strenuously. (Tetraethyl lead is lipid-soluble and is readily absorbed through skin). At least 15 deaths occurred, but the total number remains unknown. Bryson records: "As rumours circulated about the dangers of the new product, ethyl's ebullient inventor, Thomas Midgley, decided to hold a demonstration for reporters to allay their concerns. As he chatted away about the company's commitment to safety, he poured tetraethyl lead (TEL) over his hands, then held a beaker of it to his nose for sixty seconds, claiming all the while that he could repeat the procedure daily without harm. In fact, Midgley knew only too well the perils of lead poisoning: he had himself been made seriously ill from overexposure a few months earlier and now, except when reassuring journalists, never went near it." (page 151)
Gradually the evidence against leaded gasoline accumulated. In 1924, the New York Board of Health argued for a ban on sales of leaded gasoline, but the US Bureau of Mines approved its use. In May, 1925, a meeting was convened by the US Treasury Department to review the evidence. Lead supporters included the US Surgeon General and the president of the corporation that produced the leaded gasoline. The Surgeon General came under some heavy political influence and in 1928 told NYC that there were no grounds for banning TEL. In the period 1921 to 1932, Andrew Mellon was both Secretary of the Treasury, and in charge of the Public Health Service. He was also a major owner of Gulf Oil Co, which had the exclusive contract to distribute leaded gasoline in parts of the US.
A heroine steps onto the stage in the form of Alice Hamilton (1869-1970). She was trained in medicine, pathology and bacteriology and became a pioneer of industrial toxicology. In 1919 she also became the first female professor at Harvard. At the May 1925 Treasury Department meeting, she showed that lead is a cumulative poison that does not produce obvious symptoms, yet forms a significant public health issue. She declared "I am utterly unwilling to believe that the only substance which can be used to take the knock out of a gasoline engine is tetraethyl lead." Her arguments fell on deaf ears.
In 1933 further research into ethanol showed it to be equally effective. It was promoted by Henry Ford and was marketed as an additive in areas of the US Midwest. However, it failed commercially due to the dominance of the major oil companies who refused to sell gasohol in their gas stations. In 1936 90% of gas sold in the US still contained TEL. In 1943, Byers and Lord reported the continued evidence for persistent neurotoxic effects of childhood lead poisoning (i.e., cognitive deficits did not abate several years after childhood poisoning incidents); a major exposure source was from inorganic lead in the exhaust of gasoline-powered engines. This had virtually no impact on the industry, however, and in 1959 the US Public Health Service approved a request from the manufacturing company to actually increase the lead content of gasoline.
The final removal of lead from gasoline had little to do with fears about poisoning children. Following severe smog episodes, the 1970 clean air act led to auto manufacturers adding catalytic converters to reduce motor vehicle emissions (especially volatile organic carbons) that are precursors to urban smog. The converters could not operate with leaded gasoline. Even so, lawsuits from the TEL manufacturing company delayed the phase-out of leaded gasoline. They successfully influenced the National Academy of Science, which, in 1972, declared that airborne lead emissions have no known harmful effects.
Only in 1980 did the tide finally turn, and the Academy of Science acknowledged the accumulated evidence, declaring leaded gasoline to be the greatest source of atmospheric lead pollution. But the lead lobby was still influential: the proposal that gasoline be blended by 10% grain alcohol was blocked by the Reagan administration, but Congress passed legislation to outlaw oil companies' opposition to the sale of gasohol at their pumps. Even then it was still not quite dead, and in 1981 George Bush (senior) headed a task force that tried to relax the phase-out of leaded gasoline, but finally, in 1986 the EPA limited lead content of gasoline to 0.1 gm per gallon.
The result? In 1983, soon after the phase-out began, the CDC reported that use of leaded gas had fallen by 50% and that blood lead levels had fallen by 40% between 1976 and 1980. By 1994, CDC reported that blood lead levels had fallen 78% between 1978 and 1991. This has been reinforced in a 2003 report in the New England Journal.
Source: Dr. Don Wigle "Child health and the environment" Oxford University Press, 2003
The Public Health system of Canada runs in parallel (and occasionally intertwines with) the health care system. Here, Public Health refers to the preventive and population-based measures for safeguarding the health of the public (this includes population health assessment, health protection, health promotion, disease and injury prevention, health surveillance, and emergency preparedness). The health care system refers to the system of caring for individual patients (in hospital, through publically-funded health insurance, etc). The vast majority of Canadian health dollars go towards health care; public health only accounts for 1-3% of the money spent annually in Canada on health.
In Canada, one can think of the public health system as having three main levels: federal, provincial/territorial, and the Regional Health Authorities. The names and roles vary somewhat by province: in Ontario, the Regional Health Authorities are called Public Health Units. There are 36 Public Health Units in the province; click here to see a map of them.
The Ontario Public Health Units, lead by their Medical Officer of Health, work under the direction of their Board of Health (made up of local representatives, including municipal government officials) and the province. The province mandates required programs and activities identified in the Ontario Public Health Standards. The funding of Public Health Unit activities is approximately 75% transferred from the provinces and 25% provided from the municipalities served by the Health Unit.
Like Health Care, Public Health is not standardized across the provinces and territories of Canada; it is directed largely by each province/territory with consultation and coordination from the federal government. Each province/territory has a Chief Medical Officer of Health.
The federal government has a lead role in food and drug regulation, quarantine, international liaison, advanced laboratory technology and other select activities. Nonetheless, the major role for the federal government in public health is one of coordination and leadership. Numerous Expert Groups, Liaison Committees and Task Groups are made up of provincial, territorial and federal employees and organized by federal workers (employees of Public Health Agency of Canada). These groups report to the Public Health Network Council, a council of federal, provincial and territorial members; the Council reports to the Conference of federal/provincial/territorial Deputy Ministers of Health. A diagram illustrates these different groups and their relationships.
Ultimately, the federal Deputy Minister of Health reports to the Provincial and Territorial Minsters of Health, who then make decisions for their province or territory.
Additional Information on the organization of public health in Canada
Public Health Agency of Canada website
D.L. Mowatt and D. Butler-Jones. Public Health in Canada: A Difficult History
National Collaborating Centre for Healthy Public Policy – Structural Profile of Public Health in Canada
Maintaining public health requires that we detect outbreaks of infectious disease as early as possible so that the causal agents can be traced and eliminated. Surveillance serves this goal.
Surveillance involves systematic collection, analysis and interpretation of health data (or data on health determinants) to track (and forecast) health events. It also involves the dissemination of this information to authorities responsible for disease prevention and control. It is typically applied to reportable diseases, which pose a threat to public health.
Surveillance is typically used to monitor and forecast trends in new or emerging diseases, but can also be used to identify and deal with immediate situations such as contamination of public water supplies. It provides the evidence to identify outbreaks, to help plan interventions or health policies, and to evaluate their impact subsequently.
You may find it useful to think in terms of a surveillance loop, as follows:
Surveillance may be passive, in which the system waits for cases to be reported, or active, in which you search out the cases of disease. Active surveillance may be necessary with an outbreak of a new disease (such as SARS), in which physicians in the community would not routinely be reporting cases. Active surveillance might (for example) involve calling hospital staff who took sick leave from work to see whether they had symptoms compatible with SARS.
In Canada, disease surveillance is routinely undertaken by a number of provincial and federal agencies; these are loosely grouped under a federal Network for Health Surveillance.
“The Health Protection and Promotion Act (HPPA) outlines the communicable diseases that are designated reportable in the province of Ontario. Under the authority of the HPPA, Ontario Regulations 559/91, these diseases or suspected occurrences of these diseases, must be reported to the local Health Unit by physicians, laboratories, administrators of hospitals, schools, and institutions.”
- Epidemiology and Surveillance, City of Toronto)
There are two levels of reportable disease in Ontario. Out of the list below (up to date as of 2016), diseases marked * (and Influenza in institutions) should be reported immediately to the Medical Officer of Health by either telephone (24 hours a day, 7 days a week) or fax (Mon-Fri, 8:30 am – 4:30 pm only). Other diseases can be reported the next working day by fax, phone or mail.
Acquired Immunodeficiency Syndrome (AIDS) Acute Flaccid Paralysis
*Clostridium difficile associated disease
(CDAD) outbreaks in public hospitals
Creutzfeldt-Jakob Disease, all types
1. *Primary, viral
4. Subacute sclerosing panencephalitis
*Food poisoning, all causes
*Gastroenteritis, institutional outbreaks
*Giardiasis, except asymptomatic cases
*Group A Streptococcal disease, invasive
Group B Streptococcal disease, neonatal
*Haemophilus influenzae b disease, invasive
*Hantavirus Pulmonary Syndrome
*Hemorrhagic fevers, including:
1. *Ebola virus disease
2. *Marburg virus disease
3. *Other viral causes
1. *Hepatitis A
2. Hepatitis B
3. Hepatitis C
*Meningococcal disease, invasive
Paralytic Shellfish Poisoning
Pertussis (Whooping Cough)
Pneumococcal disease, invasive
*Respiratory infection outbreaks in institutions
Rubella, congenital syndrome
*Severe Acute Respiratory Syndrome (SARS)
*Verotoxin-producing E. coli infection, including Haemolytic Uraemic Syndrome (HUS)
*West Nile Virus illness, including:
*West Nile fever
*West Nile neurological manifestations
(* indicates need for immediate reporting)
Gunshots. In addition, a primary care doctor who sees a patient with gunshot or stab wounds must report this to the local police (in most provinces, including Ontario). The need to care for the wounds comes first, but reporting must be done as soon as possible without interfering with care. Self-inflicted knife wounds are not reportable.
Driving. Physicians in Ontario have a legal responsibility to report all patients with medical conditions that may impede their driving ability.
“Every legally qualified medical practitioner shall report to the Registrar the name, address and clinical condition of every person 16 years of age or over ... who, in the opinion of such medical practitioner, is suffering from a condition that may make it dangerous for such person to operate a motor vehicle.”
- Section 203 of the Highway Traffic Act
Mandatory reporting includes:
You are to report them to the Medical Review Section of the Ministry of Transportation. If you do not report and the driver gets into an accident, you may be held liable. Note, it is not the physician's role to decide whether the patient is competent to drive; that is done by the Ministry. The Regional Geriatric Program of Eastern Ontario puts out a very helpful guide to the physician's role in assessing driving competence.
The Public Health Agency of Canada publishes a list of diseases under national surveillance.
The City of Ottawa Health Department provides some basic information on public health and infectious disease control
Chronic Disease Surveillance with the Public Health Agency of Canada. This allows you to print off maps showing the distribution of all kinds of disease - go play with it!
Prevention includes interventions not only to prevent the occurrence of illness (such as risk factor reduction) but also to to arrest its progress and reduce its consequences once established.
To prevent disease we must understand the natural history of the condition, its distribution in the population, and how to detect early cases. The natural history begins with a biological onset of disease: the first lesion. This lies undetected for a varying period of time (from zero with a cut finger, to many years for some cancers), called the preclinical phase. When symptoms appear, the patient typically seeks help, perhaps leading to a diagnosis. If therapy begins, this may alter the natural history, turning it into the clinical course of the condition. If there is no treatment, the natural history continues its course, ending in cure, chronic condition, or decline. See the diagram below.
The metaphor of the "iceberg of disease" has been used to suggest that for every case that comes to a clinician, there are likely to be more people with pre-clinical disease in the community. Bear in mind that the cases of disease that you will see in your training in a university hospital represent only a small fraction of the cases who are seen by a primary care doctor and these, in turn, represent a fraction of the people with the condition.
The goals of medicine are to promote and to preserve health, to restore health when it is impaired, and to minimize suffering and distress. These goals are embodied in the word "prevention" which for simplicity is often divided into three levels. Note that this is a simplification, but it is a useful place to begin:
Primary prevention is concerned with preventing the onset of disease; it aims to reduce the incidence of disease. It involves interventions that are applied before there is any evidence of disease or injury. Examples include protection against the effects of a disease agent, as with vaccination. It can also include changes to behaviors such as cigarette smoking or diet. The strategy is to remove causative risk factors (risk reduction), which protects health and so overlaps with health promotion. A successful primary prevention program requires that we know at least one modifiable risk factor, and have a way to modify it.
Primary prevention may be aimed at individuals or at whole communities. Individual approaches (encouraging your patient to stop smoking) have the advantages that the clinician's personal contact should be motivational; the message can be tailored to the patient, and you can support him in actually making the decision to stop. But the limitation is that your advice does not tackle underlying forces driving his behaviour in the first place or the context in which his behaviour occurs (his friends may continue to smoke). Therefore, a community or population approach (e.g. via mass media advertising, increasing taxes, or banning smoking in public places) tries to change risk factors in the whole population. It is more radical and may produce cultural and contextual changes that support individual efforts.
Note that there is also a concept of "Primordial prevention" which seeks ways to "avoid the emergence and establishment of the social, economic and cultural patterns of living that are known to contribute to an elevated risk of disease" (Beaglehole, Bonita & Kjellstrom, Basic Epidemiology). This would include environmental control of disease vectors, and eliminating predisposing factors such as illiteracy and maternal deprivation and other social determinants of health (JM Last, Dictionary of Public Health).
Secondary prevention is concerned with detecting a disease in its earliest stages, before symptoms appear, and intervening to slow or stop its progression: "catch it early." The assumption is that earlier intervention will be more effective, and that the disease can be slowed or reversed. It includes the use of screening tests or other suitable procedures to detect serious disease as early as possible so that its progress can be arrested and, if possible, the disease eradicated. An example is the Pap test to screen for cancer of the cervix, or a PSA blood test for prostate cancer; other instances include teaching people about the early signs of disease that they should watch for, and what type of treatment to seek. This is the task of preventive medicine.
Screening is central to secondary prevention because it is the process by which otherwise unrecognized disease or defects are identified by tests that can be applied rapidly and on a large scale. Screening tests distinguish apparently healthy people from those who probably have the disease. To be detectable by screening, a disease must have a long latent period during which the disease can be identified before symptoms appear. This is the purpose of screening tests. Implicitly, secondary prevention is used when primary prevention has failed.
Tertiary prevention refers to interventions designed to arrest the progress of an established disease and to control its negative consequences: to reduce disability and handicap, to minimize suffering caused by existing departures from good health, and to promote the patient's adjustment to irremediable conditions. "Minimize the consequences." This extends the concept of prevention into the field of clinical medicine and rehabilitation.
The following diagram may help to clarify these levels of prevention, and to link them to the populations they each target.
Louise Russell's economic evaluation of the cost-benefit of preventive strategies
It is tempting to compare the effectiveness of preventing disease versus treating it.
An article in the American Journal of Preventive Medicine ran a mathematical simulation using the US population that compared the numbers of deaths that would be avoided, or at least delayed, under 3 management scenarios for cardiovascular disese: provide ideal treatment to every case of heart disease (including a range of diagnoses); provide complete prevention of the major risk factors prior to a first cardiac event; and intervening following a first cardiac event to remove all risk factors for a subsequent event.
Cardiac disease currently kills 44% of Americans; if perfect care were delivered, this would prevent or postpone 8% of all deaths. If ideal care were given following a first event, this would prevent or delay 23% of all deaths; if ideal primary prevention were delivered, this would prevent or postpone 33% of all deaths.
Clearly prevention (like care) would never be perfect, but this indicates the theoretical potential of prevention versus cure.
(Source: Kottke TE, et al. Am J Prev Med 2009;36(1):82–88)
An obvious criticism of medical care is that it tackles health issues without addressing their underlying determinants. The pattern of health problems can be expected to continue until someone addresses health determinants.
The cartoon illustrates the idea: the two doctors are so busy mopping up the spill that they neglect to turn the tap off.
Do you think that the same criticism might be made of secondary and tertiary prevention?
The Canadian Task Force on Preventive Health Care grew out of earlier work of the "Task force on the Periodic Health Examination." It is an advisory group that produces sets of recommendations on preventive care according to the age and sex of a patient, all based on solid evidence of effectiveness. The Task Force report is available on the web and is an indispensable resource for primary care clinicians. It covers prenatal care, pediatrics, dental prevention, immunization guidelines, and then a range of preventive approaches for specific conditions (circulatory disease, infections, neoplasms, etc.)
Link: Health Canada's Guide to Clinical Preventive Health Care
‘Screening’ is the process of detecting disease early, with the intention of intervening to halt its progression. This is linked to the idea of secondary prevention: interventions to stop further decline one a disease has begun but before symptoms appear. Secondary prevention requires a way to proactively detect disease as early as possible, perhaps even before the patient is aware of it. This is the role of screening.
Screening is an initial examination; it is usually not diagnostic and requires appropriate investigative follow-up and treatment.
Screening is justified when early intervention is more successful and cost-effective than waiting until symptoms appear and then treating the condition. Examples of screening tests include mammography for breast cancer, pre-natal amniocentesis for detecting congenital malformations, or psychological tests to identify early signs of cognitive decline.
The safety of screening tests is most important, because screening is initiated by the health service rather than by the people being screened. Screening tests should be cheap and simple to administer - they need to be applied to large numbers of people. Hence, screening tests are generally not invasive (and because of this may not be adequate to offer a definitive diagnosis). As they are administered early in the course of a disease, it may not be possible yet to provide a definitive diagnosis (they may suggest that something is going on, but it's too early to tell exactly what). In addition, the tests are often designed to be inclusive, to capture a broad array of possible cases, along with the probable and actual cases. In the jargon, they are designed to be sensitive, rather than specific (see below). The non-cases can then be separated out during a more detailed clinical assessment.
There are different types of screening, each with specific aims
The following criteria should be met as best as possible before a screening program is instituted:
1. Regarding the Disease:
- The disease should be serious - e.g., it causes death, disability, or discomfort;
- The natural history should be understood;
- The disease must have a latent period during which it can be detected before symptoms appear;
- The latent period between first signs and overt disease should be long enough that screening significantly advances the detection of disease.
2. Regarding the Diagnosis and Treatment
- There is an available, effective, acceptable, and safe treatment;
- Early treatment should be more effective than later;
- Facilities need to be adequate.
3. Regarding the Screening Test
- Should be sensitive (and ideally also specific), and have good predictive value (see below);
- Simple and cheap;
- Safe and acceptable;
- Should be reliable.
Sensitivity, Specificity and Predictive Values
A screening test should be accurate: ideally it should neither miss cases (thereby giving false reassurance), and nor should it falsely classify healthy people as diseased. (What may be the results of such "labelling"?) These aspects of accuracy reflect the validity of the test.
Sensitivity is the proportion of truly diseased persons in the screened population who are identified as diseased by the screening test. Sensitivity is a measure of the probability of correctly identifying a case, or the probability that any given case will be identified by the test (Syn: true positive rate). You can remember the term "sensitivity" because being sensitive to something means you can detect it.
Specificity is the proportion of persons without the disease who are classified as such by the screening test. It is a measure of the probability of correctly identifying a non-diseased person with the screening test (Synonym: true negative rate). To help you remember the term, a specific test is one that picks up only the disease in question, so has a narrow focus, which explains the term 'specific'.
Sensitivity and specificity only tell us how many people will be correctly classified by a screening test. But no screening test is perfectly sensitive and specific, so your patient with a positive test result will want to know how likely it is that he or she actually does have the disease. These are the predictive values of the test.
Predictive value: The likelihood that someone with a positive score on the test really does have the disease is called the "predictive value of a positive test result," or "positive predictive value," or PPV for short. The equivalent measure for a negative result is the “negative predictive value” or NPV.
Another way to think of this is that the positive and negative predictive values are the proportions of positive and negative results that are true positive and true negative results.
For a fuller explanation, click here to go to the page “Criterion Validity (Sensitivity, Specificity, PPV, NPV, LRs)” in the Research Methods theme.
Overview of screening (ppt slides) by Dr. Ann Jolly: a useful summary, with examples.
The following table was prepared by the Ontario Injury Prevention Resource Centre. While it allows you to see the rank for various outcomes (hospitalizations, deaths, etc), it also gives an overall impact ranking, across all outcomes.
More information on injury prevention can be found at the (Ontario Injury Prevention Resource Centre).
Based on 2009-2010 data, Statistics Canada reported that approximately 15% of the Canadian population suffers an injury each year that is “severe enough to limit their usual activity”. The likelihood and type of injury varies by age group. Young people aged 12-19 have the highest likelihood of injury, followed by working age adults (20-64) and then seniors (64+). Many (66%) of injuries in 12-19 y.o. were linked to sports, while 47% of the injuries of working adults were linked to sports and work. Seniors most often got injuries while walking or doing household chores.
Falls are the leading cause of injury; in 2009-2010, 63% of seniors, 50% of adolescents and 35% of working-age adults were injured in falls.
William Haddon made a useful contribution to thinking about how injuries occur and how to prevent them. The "Haddon Matrix" shows the host, agent and environmental factors involved, set against the time sequence of an incident. (Link to Agent–Host–Environment model)
The cells of the matrix illustrate the range of risk or protective factors involved; Haddon emphasized the multidisciplinary nature of potential interventions.
To illustrate, for a motor vehicle collision:
|Agent or vehicle||Physical environment||Social environment|
(→ primary prevention)
(in a rush to get home?);
|Car design & handling;
Anti-lock brakes, etc;
Maintenance of car
|Reliance on private, rather than public transportation raises traffic load;
Compliance with seatbelt laws
|During the event
(→ secondary prevention)
|Wearing seatbelt?||Air bags working?
Size of car & crash resistance
ice on road?
|Quality of emergency assistance;
Assistance from bystanders
(→ tertiary prevention)
|Ability to call for help
Knows first aid?
|Tendency of car to catch on fire||Emergency vehicle access to collision site||Continued funding for emergency services|
Haddon W. A logical framework for categorizing highway safety phenomena and activity. J. Trauma 1972;12:193-207.
Note how the matrix links neatly to the stages of prevention. In this context, primary prevention refers to interventions before the event, to prevent it completely; secondary prevention involves lessening the extent of injury given that an event occurs (e.g., wearing a seatbelt will not prevent the collision but may lessen its effects). Tertiary prevention limits the subsequent difficulties a person encounters given his injuries (rehabilitation, etc.). You could also add primordial prevention (not shown above), which would refer to underlying social change that would alter the circumstances in which the accident occurred. For example, encouraging public transportation would reduce crowding on the roads, likely preventing some accidents.
The Source - Path - Receiver model offers a useful guide to intervention points to prevent industrial injuries. It is a bit simpler than the Haddon model but also useful. Interventions either modify the source (machine or chemical). Some approaches are mentioned in the diagram. Where this is not possible (or in addition), interventions can break the pathway between the source and the person. A third approach is to work on shielding the worker - examples include hearing or eye protection, or simply moving the person away from the source of danger.
More from Haddon
Haddon also proposed a generic sequence of ten countermeasures to reduce the risk of injuries (very similar to the Source-Path_receiver model). This can be applied to many types of events or injuries, and it also covers prevention in general. Here we have applied it to smoking:
Countermeasure Example (reducing smoking-related diseases) 1. Prevent the creation of the hazard Eliminate cigarettes 2. Reduce the amount of hazard brought into being Reduce tobacco growing by subsidising alternative crops 3. Prevent the release of the hazard Forbid tobacco sales to minors 4. Modify rate of release of the hazard Develop cigarette that burns slowly 5. Separate hazard from person being protected by time and space Limit times that vending machines are open 6. Separate hazard from person being protected by a physical barrier Filters on cigarettes 7. Modify basic qualities of the hazard Reduce nicotine content of cigarettes 8. Make what is to be protected more resistant to hazard Limit exposure to synergistic causes (e.g., environmental carcinogens) 9. Counter damage done by hazard Screening program to detect early cancers 10. Stabilize, and repair damage Provide good health care for cancer patients
The Canadian Hospitals Injury Reporting and Prevention Program (CHIRPP) is a computerized information system that collects data from 10 pediatric hospital emergency departments and from 6 general hospitals in Canada. The information is analyzed to show patterns and thereby identify hazards and possible ways to prevent injuries. It publishes a monthly newsletter, and has a web site.
While injuries are still the number one cause of mortality in young people, mortality from all causes, and especially from injuries, has been declining steadily among young people in Canada. This is a success story - with the notable exception of suicide.
These figures refer to Canadian males age 15-19 years, 1981 to 1997; the rates are per 100,000 (Source CHIRPP Newsletter, December 1999)
Males aged 15 - 19 years
All cause mortality
Deaths due to injuries
Motor vehicle collisions
The rate of death by suicide (per 100,000) in males 15-19 was 13.75 in 2007 and 12. 6 in 2009 (some decrease) – however, it increased again to 14.1 in 2012. Some decrease can be seen since the 1980s, but not at the same rate as other causes of mortality. In 2011, there were 198 deaths in Canada by suicide in individuals aged 15-19: 140 for boys and 58 for girls.
Many more self-harm cases do not end in death. From 2009-2010 to 2014-2015, the rate of intentional self-harm– related hospitalizations in girls (aged 10-19) has increased from 78 to 164 per 100,000 female youths (increased by >110%), while the rate for boys has increased from 23 to 32 per 100,000 male youths (increased by >35%).
For more information, please see this handout by the CIHI “Intentional Self-Harm Among Youth in Canada”).
For more injury on child safety and injury prevention, go to Child Safety Link.
Health promotion: The process of enabling people to increase control over their health and its determinants, and thereby improve their health. It includes encouraging healthy behaviors (e.g., via smoking legislation), building healthy environments (bicycle paths, etc).
Health promotion does not target a single risk factor or disease, but tries to enhance each person’s ability to improve their own health. A central goal is to create environments that support healthiness, from which came the metaphor of ‘upstream’ and ‘downstream’ interventions.
A passer-by sees a body floating down a river, and calls 911. Firemen arrive and haul the body out, and paramedics start resuscitation and rush the victim to the ER where the hospital PR director proudly announces that the highest quality care is being delivered to deal with the situation. By contrast, health promotion would focus upstream to figure out what is causing people to fall into the water, and correct it. Health promotion aims to prevent disease by modifying the social determinants of health or ‘upstream’ factors.
Beyond preventing disease, health promotion also seeks to create a context in which health can evolve spontaneously. Health promotion often seeks to support neighbourhood groups in taking charge of local issues that affect health: food choices in the school cafeteria; setting up exercise groups; arranging for volunteers to assist elderly people, etc.
Health protection: more specific than health promotion, this targets particular health hazards and reduces people's exposure to them. Drug safety regulations; food inspections; hazardous substances and pollution; vector control and smoke-free spaces are examples.
For example, health protection can refer to “ensuring safe food and water supplies, providing advice to national food and drug safety regulators, protecting people from environmental threats, and having a regulatory framework for controlling infectious diseases in place. Ensuring proper food handling in restaurants and establishing smoke-free bylaws are examples of health protection measures." (Health Canada, 2005).
You can think of health promotion, health protection and disease prevention in relation to each other. Health promotion encourages healthy behaviours and living styles (increase positives), health protection targets general health hazards (decreases negatives) and disease prevention targets particular risk factors to halt the development or consequences of specific diseases (decreases targeted negatives).
The Ottawa Charter for Health Promotion: In November, 1986, Ottawa was the venue for the First International Conference on Health Promotion, organized by the World Health Organization (WHO). The Ottawa Charter was an international agreement signed at this Conference that proposed action "to achieve health for all" by the year 2000. It included the following strategies:
The Charter also identified seven prerequisites for adult health: peace, shelter, education, food, income, a stable eco-system, and sustainable resources
Click here for “Health promotion, the Ottawa Charter and ‘developing personal skills’: a compact history of 25 years”, a 2011 paper by Mcqueen and De Salazar reflecting in the impact of the Ottawa Charter over the last few decades.
Discussion of health promotion continues at an international level. The most recent Global Conference on Health Promotion was hosted in Finland in 2013.
From the WHO website:
“The main theme of the conference was “Health in All Policies” (HiAP) and its focus was on implementation, the “how-to”. It was structured around six themes.
The conference aimed to:
Clinicians find that altering patients' health behaviour (helping them to reduce health hazards (like smoking) or increase positive health behaviours (like exercise)) is agonizingly slow and difficult. You will be only one of many factors that influence your patients' behaviour.
Psychology offers several "theoretical models" that identify personal and situational factors likely to influence health behaviour. Most influences tend to maintain behaviours, but some of the models propose ways you can change this stability.
Health Belief Model (HBM): one of the earliest and best known of the health behaviour models. Here, three factors explain whether or not a person will follow a recommendation to change their behaviour:
A second well-known model is the Theory of Reasoned Action (TRA). Again, behaviour reflects three pressures, in this case the person's attitudes toward the recommended action, the feeling of social pressure towards acting, and their confidence in being able to control their behaviour.
The HBM and TRA list the reasons why a person may act as they do. Clinicians are interested in changing health behaviour, which introduces the "stages of change" model:
Stages of Change Model: This theory was formulated by Prochaska and Di Clemente, originally for smoking cessation. It is sometimes called the "trans theoretical model", or TTM. It states that at any time, for any behaviour, a person is in one of the following stages of behaviour change:
The stages of change shown as a continuum, around which a patient may circle as they seek a lasting change to a health behaviour
The five-stage model applies mainly to individual patients. When you get involved with change at the level of a whole community, program planning may consider 4 E's: Education, Environmental supports, Economic levers, and Enforcement of regulations and legislation. For example, a smoking prevention program might involve health education; changing norms in the social environment via smoking bans in public places; price controls through taxation; and laws to restrict access to tobacco for young people. (Source: C.P. Shah: Public Health and Preventive Medicine in Canada, 2003, p27).
Health Risk Appraisal (HRA) refers to an approach to promoting healthy behaviors via providing a patient with factual evidence of how their current lifestyle may be damaging their future health.
HRA originated in work by Robbins and Hall in the early 1970s. They applied actuarial procedures to estimate the likely impact of current behaviors, health status and previous health history on a person's risk of dying prematurely. This was the cornerstone of their idea of practicing "prospective medicine" by which they meant that physicians should treat not only current illnesses, but should anticipate and forestall the development of future conditions.
The patient completes a computerized questionnaire that covers demographics, current health habits, and a brief family health history (e.g., what your parents died from). The system also requires data on BP, lipid levels, height, weight, etc. These data are processed using risk equations derived from epidemiological studies that estimate the person's risk of premature death; typically the estimate covers risk of dying from the 10 commonest causes of death over the coming 10 years. This risk is sometimes expressed as an equivalent health age: if a 50 year-old smokes and is overweight, he may be equivalent in health expectancy to a 55 or even a 60 year-old. To indicate the impact of each adverse health habit, the system then calculates the extent to which the person could improve their "health age" if they were to quit smoking, lose weight, etc.
In theory, HRA should motivate change because it gives personalized information, it shows the improvement that could occur with altered lifestyle, it is precise, and is delivered in a health care setting in which support can be provided to help the person change. HRA is therefore an adjunct to health promotion.
Examples of risk appraisal systems are easily found on the Web: search for "Health Risk Appraisal." The Wellsource company has a lot of information on its web site; you may find this useful for patients who need to change their lifestyle.
Lawrence Green is a health educator who wanted to broaden our outlook on health and the factors that influence it. He moved beyond the "medical model" (bacterium → sick patient → antibiotic → well patient) to emphasize the social context of illness and consider the environment in which people live. He also argued that the ultimate goal is not merely to treat an illness, but to enhance quality of life.
Precede: Green argued that we should look at underlying factors that "precede" or determine a patient's quality of life. This is the first part of the assessment sequence. Green proposed assessments to make (or at least questions to ask) to diagnose the person's condition (or the condition of a group, such as stressed-out students).
Proceed: The second part of the loop asks the question "how do we evaluate what we have done?" This reminds us that it may not be sufficient to record just the final outcome. There are various stages of evaluation: what was done? (implementation); how it was undertaken? (process evaluation); what intermediate variables were changed? (impact) and what was the final result? (outcome evaluation). These steps in the evaluation help to show what may have gone wrong if the outcome does not work out as expected.
As a clinician, you should have a general understanding of the approaches used in behavior modification. This is a set of techniques for altering a person's behaviors, and their reactions to stimuli, through positive reinforcement of desired behaviors, and negative reinforcement of undesirable behaviors. Some of the core ideas:
Reinforcement: Positive rewards, such as praise, encouragement, or actual rewards (linking a child's allowance to keeping their room tidy). Reinforcement must follow the desired behavior immediately, as delay may lead to reinforcing the wrong behavior. A rough ratio of 5:1 positive to negative reinforcement is often recommended: it seems most effective to praise and encourage five times as often as you criticize or punish.
Extinction: If the environment ignores a behavior, it will (eventually!) go away. Often you will see parents inadvertently reinforcing their child's whining by paying close attention to them and solicitously asking if everything is OK. Ignoring it, or suggesting that the child "use words" may lead to the child abandoning whining as a way to get attention. Extinction may be most effective if it is linked to reinforcement of the desired behavior ("I love it when you use your words!")
Shaping: This creates a desired behavior that may not exist by reinforcing an existing behavior that closely resembles the desired behavior. You then demand a more desirable behavior each time before offering the reward. So, with the messy bedroom, encourage picking up one sock, then next day expect both socks to be picked up, then the shirt as well, and so on.
Intermittent Reinforcement: You do not want the behavior to occur only to obtain reinforcement. So, set a schedule to reward desired behaviors, either in a fixed ratio (e.g. reward when they keep their room tidy for the whole week), or on a variable ratio (e.g., offer great encouragement from time to time: "I have noticed that for the past while you have been keeping your room very tidy. That's really good!") Of course, people vary as to how long they are willing to keep up a behavior without reward. This ability to defer gratification, even in very early childhood, has been found to predict a range of successful outcomes in adult life. (See, for example, Mischel W, Shoda Y, & Rodriguez ML (1989). Delay of gratification in children. Science, 244, 933-938).
“ABC's” As a clinician advising a patient (e.g., the distressed parent of a kid who is acting out), your history taking may consider the Antecedents of the behavior: what sparks it? The Behavior itself: what, exactly is the child doing? And the Consequences: when the behavior occurs, how do other people - the mother - react?
A book available on the internet reviews health behaviour theories: "Theory at a glance: a guide for health promotion practice"
Motivational interviewing is an approach to patient counseling that helps patients to make changes that will improve their health. It offers you techniques for helping a patient move across the stages of change outlined above. It is often applied in dealing with addictions. It seeks to build the patient's own intrinsic motivation working from a client-therapist relationship; this focuses mainly on areas of ambivalence and resistance.
The five general principles of motivational interviewing may be summarized using the mnemonic 'DARES':
Develop discrepancy (help the patient see that there's a problem in their behavior; How does their current behavior compare to their ideal?) Acknowledge the positives and negatives in changing the behavior, but create some dissonance in their mind. Are there negative consequences of the behavior (financial, marital, social)? Once the client perceives some discrepancy between behavior and their values, focus on this until they see a reason to change. Restate any discrepancies that you perceive in the patient's presentation. (See the Contemplation stage of the stages of change model.)
Avoid argumentation or confrontation over the behavior. Argumentation causes people to reinforce their position and resist change (you are there, after all, to help them). Keep on the patient's side. Don't force them to accept a label such as "alcoholic" if they are not willing to do so. Power struggles are not helpful. It has to be the client who expresses reasons to change.
Roll with resistance. Do not argue, but try to clarify your understanding of their position. If you are not making progress and the client is resisting change, it may be time to take a different approach. Perhaps they perceive the situation differently than you do; go back to try and understand their viewpoint. Remain non-judgmental; be respectful to raise the chance they will stay engaged. Invite them to consider new perspectives.
Express empathy – identify with their perceptions and feeling states and their reasons for the behavior. Create a climate of change by building trust.
Support self-efficacy. Encourage their perception of themselves as a capable person. Affirm their positive statements. Reinforce the patient's role as a problem-solver. Remind them of past successes; Give hope.
Updated August 17, 2017