A screening can yield two results:
- abnormal: the risk factor or disease was detected;
- normal: the risk factor or disease was not detected.
In the case of an abnormal result, you will be referred to a healthcare provider for further investigation and follow-up. Often, another diagnostic test follows to determine with more certainty whether the risk factor or disease is actually present.
The result of an examination or test is never 100% reliable and can sometimes give a false positive. There are two types of false results:
- false positive: the result of the test is abnormal, but further investigation shows that there is nothing wrong after all. So in this case, the screened person was worried for no reason and interventions that were unnecessary may have taken place;
- false negative: the result of the test is normal, but it later turns out that something was wrong after all. The detected risk factor or disease is present after all. In this case, the screened person was wrongly reassured and there is a possibility that the person’s health deteriorated while this was avoidable.
False positives or false negatives cannot be completely ruled out in any test. The quality of a screening test depends mainly on its sensitivity and specificity.
The sensitivity of a test reflects how well the test avoids false negative results. A test with a sensitivity of 99% yields an abnormal result in 99% of all screened persons who have the detected risk factor, pre-stage or disease. Only in 1% of these individuals is the result then a false negative.
The specificity of a test reflects how well the test avoids false positive results. A test with a specificity of 99% yields a normal result in 99% of all screened individuals who do not have the detected risk factor, pre-stage or disease. The result is then a false positive in only 1% of these individuals.
Ideally, a screening test should have both a sensitivity and a specificity of 100%. In practice, however, a good compromise between the two properties is usually the highest achievable objective.