Screening is the application of a test to asymptomatic individuals to assess the likelihood that an individual has a particular disease. Therefore, screening is done on large populations and the test being used should be suitable to be applied to a large number of people. It should pick up all individuals who have the disease (high sensitivity). Furthermore, the test should be simple, cheap and minimally invasive. Obviously, the disease screened for should be serious enough to make subsequent treatment necessary.

Individuals with a positive test require further tests to make a definite diagnosis. For example, a heel prick blood test is being used to diagnose phenylketonuria (an autosomal recessive hereditary condition that if left untreated results in severe mental retardation). Further genetic testing after a positive screening test is required.

It is possible that an individual tested positive on the screening test, but after further testing has been show not to have the disease. This is called a false positive result.

Once people have been diagnosed as having the disease following a screening test, treatment should be available, otherwise there would have been no point in performing the screening in the first place! In the example above, omitting phenylalanine from the diet can treat phenylketonuria.