Which of the following is NOT one of the three ways to increase the positive predictive value of a test?

• A. test animals known to be diseased
• B. use more than one test
• C. increase sample size
• D. use a more specific test

Answer: (C)

Positive predictive value is the probability that a positive test result truly indicates disease. PPV can be increased by having a higher disease prevalence in the tested group, by using a test with high specificity (fewer false positives), or by confirming a positive result with an additional test (which reduces the chance that a positive is a false positive). Testing animals known to be diseased effectively raises the pre-test probability in the group, which boosts PPV. Using more than one test provides confirmation and further increases PPV by lowering false positives. Using a more specific test directly reduces false positives and increases PPV. Increasing sample size, however, does not change the PPV itself; it affects the precision of the PPV estimate (narrower confidence intervals) but not the probability that a positive result is truly positive.