Negative predictive value (NPV) is the probability that a negative test is actually negative.
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 900 people are negative on the test.
We can calculate the VPN which is induced by this test:
Thus among the 900 negative people in the test, we hope for a value of 899 negative people.