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Screening for Disease   To Prevention theme page

1. Core Knowledge:

’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 justified when early intervention is more successful and cost-effective than passively waiting until symptoms appear and then just 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.

Formal Definition:
Screening is the process by which unrecognized disease or defects are identified, using tests which can be applied rapidly and on to large numbers of people.  Screening tests distinguish apparently healthy people from those who probably have the disease.  Screening is an initial examination; it is usually not diagnostic and requires appropriate investigative follow-up and treatment. 

The UK National Screening Committee defined screening in the following terms:

"Members of a defined population, who may not know they are at risk of a disease or its complications, are asked a question or offered a test to identify those who are more likely to be helped than harmed by further tests or treatment"

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 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 before a screening programme is instituted:

The DISEASE

  1. The disease should be serious - e.g., it causes death, disability, or discomfort;
  2. The natural history should be understood;
  3. The disease must have a latent period during which it can be detected before symptoms appear (see the diagram on the natural history of disease). Examples include pre-cancerous lesions;
  4. The latent period between first signs and overt disease should be long enough that screening significantly advances the detection of disease.

DIAGNOSIS AND TREATMENT

  1. Facilities need to be adequate;
  2. There is an available, effective, acceptable, and safe treatment ;
  3. Early treatment should be more effective than later.

The SCREENING TEST

  1. Should be sensitive (and ideally also specific), and have good predictive value (see below);
  2. Simple and cheap;
  3. Safe and acceptable;
  4. Reliable.

Links:   See more information of the evaluation of a screening program
              Overview of screening (ppt slides) by Dr. Ann Jolly: a useful summary, with examples.

 

SENSITIVITY, SPECIFICITY, PREDICTIVE VALUE.

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 are "validity." 

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 diagnosing 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

Specificity is the proportion of persons without the disease who are identified as such by the screening test.  It is a measure of the probability of correctly identifying a non-diseased person with a 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'. 

The relationships are often shown in a fourfold table, in which the letters a, b, c, and d represent the quantities specified below the table. To view table and equations, click here.

Link: More detail on sensitivity & specificity

 


3. Additional Information:

 

Nerd's Corner: Choosing amniocentesis
 

Clinicians often suggest screening for Down syndrome and other genetic anomalies for older women as the risk of anomalies rises sharply with maternal age. Because amniocentesis may lead to miscarriage, it is often not suggested at younger ages. A dynamic modeling approach to choosing whether or not to recommend amniocentesis suggests, however, that his approach may be incomplete. Like the benefits, the costs of amniocentesis also rise with age. The later a woman postpones birth, the greater the risk of failing to conceive before menopause, so if amniocentesis does cause a miscarriage the woman may miss a critical opportunity to conceive. By contrast, the costs of miscarriage are lower at younger ages, because the mother has time to conceive again. See "An economic model of amniocentesis choice." Fajnzylber E, et al. Advances in Life Course Research 2010; 15(1):11-26. (No: I have no idea how to pronounce that name; I just hope I typed it correctly!)

 

Yogi Berra: "You can observe a lot by just looking"