Who is being tested changes the positive/ negative predictive values of a test

When clinicians read about either a screening or diagnostic test, we are very interested in knowing:

1. If the test is positive for X (whatever it tests for), how likely is it that the test is right and X is really present? This is called the positive predictive value of the test.

2. If the test is negative for X (whatever it tests for), how likely is it that the test is right and X is really absent? This is called the negative predictive value of the test.