Screening

Disease

The disease should represent a significant public health problem because of its consequences, numbers affected, or both. Additionally, the natural history of the disease must include a preclinical detectable[Page 942]period. This is a period of time when it is feasible to identify early disease using some existing test, but before the onset of symptoms that would otherwise lead to diagnosis. This preclinical detectable period should be of long enough duration to be picked up by tests at reasonably spaced time intervals. This is why screening is generally applied for chronic conditions that develop slowly. In an acute disease with rapid onset, even if a short preclinical detectable period exists, testing would need to be extremely frequent to pick it up before symptomatic disease developed. Finally, there should be a treatment for the disease available that improves outcome and is more effective when given earlier.

Screening Test

Practically, the screening test should be relatively simple and quick to carry out, acceptable to the population receiving it, and not prohibitively expensive. It should also be sensitive, specific, and reliable. Having high sensitivity means that there will be few cases of the disease that get ‘missed’—and miss the opportunity for early treatment. High specificity means that there are few false positives—that is, people without the disease who may undergo further testing and worry needlessly. Although both are ideally high, a trade-off between sensitivity and specificity may occur, especially for tests, such as a blood glucose test for diabetes, where a positive or negative result is determined by dichotomizing a continuous measure. The sensitivity and specificity can only be determined if there is an accepted ‘gold standard’ diagnostic test to define true disease status. Although considered properties of the test, sensitivity and specificity can differ according to population characteristics such as age.

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