Screening, in medicine, is a strategy used to identify disease in an unsuspecting population. Unlike most medicine, in screening, tests are performed on those without any clinical indication of disease.
The intention of screening is to identify disease in a community early, thus enabling earlier intervention and management in the hope to reduce mortality and suffering from a disease. Although screening may lead to an earlier diagnosis, not all screening tests have been shown to benefit the person being screened; overdiagnosis, misdiagnosis, and creating a false sense of security are some potential adverse effects of screening. For these reasons, a test used in a screening program, especially for a disease with low incidence, must have good specificity in addition to acceptable sensitivity.
Examples of screening
A skin test called the PPD test is widely used to screen for exposure to tuberculosis. Health care providers may screen for depression using questionnaires such as the Beck Depression Inventory. Alpha-fetoprotein screening is used in pregnant women to help detect certain fetal abnormalities. Cancer screening is an attempt to diagnose cancer in its early stages, such as using the Pap smear to detect cervical cancer, or mammography to detect breast cancer.
In the United States, most public school systems screen students periodically for hearing and vision deficiencies, dental problems, and spinal/posture issues such as scoliosis.
Medical equipment used in screening
Medical equipment used in screening tests is usually differentiated from equipment used in diagnostic tests; in that screening tests are used only to indicate the possibility or probability of a disease or condition; whereas diagnostic medical equipment is used to make quantitative physiological measurements used in determining the specific treatment or progress of the disease or condition. Medical screening equipment is usually calibrated to a lower standard than diagnostic-level equipment; or, indeed, is often not capable of the level of precision of diagnostic equipment.
Adverse effects of screening
Like any medical test, the tests used in screening are not perfect. The test may appear positive for those without disease (false positive), or may miss people who have the disease (false negative). Even with a correct result, other factors may mean that a screening test is not beneficial to a population.
- Stress and anxiety caused by a false positive screening result.
- Unnecessary investigation and treatment of false positive results.
- Prolonging knowledge of an illness if nothing can be done about it.
- A false sense of security caused by false negatives, which may even delay final diagnosis.
- Overuse/waste of medical resources.
- Unnecessary and uncomfortable procedures looking for a disease that is unlikely.
Analysis of screening
To many people, screening instinctively seems like an appropriate thing to do, because catching something earlier seems better. However, no screening test is perfect. There will always be the problems with incorrect results and other issues listed above.
Before a screening program is implemented, it should ideally be looked at to ensure that putting it in place would do more good than harm. The best studies for assessing whether a screening test will increase a population's health are rigorous randomized controlled trials.
When studying a screening program using case-control or, more usually, cohort studies, various factors can cause the screening test to appear more successful than it really is. A number of different biases, inherent in the study method, will skew results.
Lead time bias
By screening, the intention is to diagnose a disease earlier than it would be without screening. Without screening, the disease may be discovered later once symptoms appear.
Even if in both cases a person will die at the same time, because we diagnosed the disease early with screening, the survival time since diagnosis is longer with screening. No additional life has been gained (and indeed, there may be added anxiety as the patient must live with knowledge of the disease for longer).
Looking at raw statistics, screening will appear to increase survival time (this gain is called lead time). If we do not think about what survival time actually means in this context, we might attribute success to a screening test that does nothing but advance diagnosis.
Many screening tests involve the detection of cancers. It is often hypothesized that slower growing tumors have better prognosis than tumors with high growth rates. Screening is more likely to detect slower growing tumors (due to longer pre-clinical sojourn time), which may be less deadly. Thus screening may tend to detect cancers that would not have killed the patient or even been detected prior to death from other causes.
Not everyone will partake in a screening program. There are factors that differ between those willing to get tested and those who are not.
If people with a higher risk of a disease are more eager to be screened, for instance a woman with a family history of breast cancer joining a mammography program, then a screening test will look worse than it really is. This is because there's going to be more people with the illness joining, and a higher chance of people dying of that illness.
Selection bias may also make a test look better than it really is. If a test is more available to young and healthy people (for instance if people have to travel a long distance to get checked) then fewer people in the screening population will get ill, and the test will seem to make a positive difference.
Screening may identify abnormalities that would never cause a problem in a person's lifetime. An example of this is prostate cancer screening. It has been said that "most men die with prostate cancer, not of it". Autopsy studies have shown that a high proportion of men who have died in other ways, have prostate cancer when the prostate is examined under a microscope.
Aside from issues with unnecessary treatment (prostate cancer treatment is by no means without risk), overdiagnosis makes a study look good at picking up abnormalities, even though they are sometimes harmless.
The only way to completely avoid these biases is to use a randomized controlled trial. These need to be very large, and very strict in terms of research procedure. It is not quick to do this type of research, and it is often expensive.
Principles of screening
There are a lot of issues involved with screening a population. Although some screening is not beneficial, a lot of screening is very good at increasing the health of a population by early detection of disease.
Various groups have come up with screening principles, that a test and condition will ideally fulfill before a program is evaluated.
World Health Organization — Principles of Screening
World Health Organization guidelines were published in 1968, but are still applicable today.
- The condition should be an important health problem.
- There should be a treatment for the condition.
- Facilities for diagnosis and treatment should be available.
- There should be a latent stage of the disease.
- There should be a test or examination for the condition.
- The test should be acceptable to the population.
- The natural history of the disease should be adequately understood.
- There should be an agreed policy on who to treat.
- The total cost of finding a case should be economically balanced in relation to medical expenditure as a whole.
- Case-finding should be a continuous process, not just a "once and for all" project.
- Wilson JMG, Jungner G. Principles and Practice of Screening for Disease. WHO Chronicle 1968;22(11):473 (large pdf). (See also ganfyd article.)
- UK National Screening Committee. Criteria for appraising the viability, effectiveness and appropriateness of a screening programme. 2003. Last accessed January 15, 2007.
- False positive
- False negative
- Fetal screening
- Medical test
- Newborn screening
- Prostate cancer
- Genetic screen