Whenever we create a test to screen for a disease, to detect an abnormality or to measure a physiological parameter such as blood pressure (BP), we must determine how valid that test is—does it measure what it sets out to measure accurately? There are lots of factors that combine to describe how valid a test is: sensitivity and specificity are two such factors. We often think of sensitivity and specificity as being ways to indicate the accuracy of the test or measure.
In the clinical setting, screening is used to decide which patients are more likely to have a condition. There is often a ‘gold-standard’ screening test—one that is considered the best to use because it is the most accurate. The gold standard test, when compared with other options, is most likely to correctly identify people with the disease (it is specific), and correctly identify those who do not have the disease (it is sensitive). When a test has a sensitivity of 0.8 or 80% it can correctly identify 80% of people who have the disease, but it misses 20%. This smaller group of people have the disease, but the test failed to detect them—this is known as a false negative. A test that has an 80% specificity can correctly identify 80% of people in a group that do not have a disease, but it will misidentify 20% of people. That group of 20% will be identified as having the disease when they do not, this is known as a false positive.
Screening is the process of using screening test for apparently normal group (either free or in the pre symptomatic stage).
Screening is useful for: Early detection of disease, Protection of public health, Pre placement examinations, Insurance purposes.
Screening test is a simple & cheap way of testing applied for an apparently normal group to scrutinize the group into those who probably have & those who probably have not the condition.
Screening test is required to be simple, cheap, rapid, acceptable, reliable & valid.
Reliability (repeatability) is the ability of the test to give the same result if performed in another places and / or times and by other persons.
Validity: Is the ability of the screening test to measure or discover what is required. It is the ability of the test to pick up the positives & to exclude the negatives. Validity includes sensitivity and specificity.
Results of screening test can be one of the following: True + ve: The result is +ve by both screening and confirmatory tests, True – ve: The result is – ve by both screening and confirmatory tests, False + ve: The result is +ve by the screening test and is – ve by the confirmatory test, False – ve: The result is – ve by the screening test and is + ve by the confirmatory test.
Sensitivity: Is the ability of the test to detect the positives. Test with very high sensitivity means no or very few false – ve cases.
Specificity: is the ability of the test to identify the healthy people (true – ve). Screening test with very high specificity means no or very few false positives