The figure explains the sequence and key figures of a diagnostic test as a pre-probability before the test is used in routine and as a post-probability afterwards.
A diagnostic test is a diagnostic instrument with a binary outcome (yes/no, sick/healthy, positive/negative).
Before being used in diagnostics or screening, the two parameters of the diagnostic test sensitivity and specificity are determined. This is done on (known) sick or infected and healthy or non-infected persons. In the case of sick persons, the statistical measures of true-positive and false-positive are also obtained. In the case of healthy persons, the statistical measures true-negative and false-negative are also obtained.
After the test has been performed, the positive predictive value indicates the probability that, if the test result is positive, the person is actually sick or infected (post-test probability).
After the test has been carried out, the negative predictive value indicates the probability that, in the case of a negative test result, the person is actually healthy or non-infected (post-test probability).