The positive predictive value (PPV) is the probability that a positive test is actually positive.

The predictive value is a function of the sensitivity and specificity of a test and the prevalence of the study condition. It is defined according to the following formula:

A serological test is performed on a population of 1000 people.

This test has sensitivity of 99% and specificity of 90%. 10% of the population tested is positive in the serological test.

We deduce that 100 people are positive on the test.

We can calculate the VPP which is induced by this test:

Thus among the 100 positive people in the test, we expect a value of 52 positive people.