Due to an effective prevention program, the prevalence of an infectious disease in a community has been reduced by 90%. A physician continues to use the same diagnostic test for the disease that she has always used. How have the test's characteristics changed ?: (A) Its sensitivity has increased, (B) Its positive predictive value has increased, (C) Its negative predictive value has increased, (D) The test's characteristics have not changed
Answer is C. PPV and NPV of a screening test depends on:


Sensitivity
Specificity


Prevalence of disease in the population
In this question, since a physician continues to use the same diagnostic test for the disease that she has always used, sensitivity and specificity of the test will remain same
But predictive value of a test (PPV and NPV) depends on prevalence of a disease in a population
PPV is directly proportional to prevalence of disease in the population

PPV α Prevalence of disease

As the prevalence of a disease increases in a population, PPV increases for the screening test
NPV is inversely proportional to Prevalence of disease in a population

NPV α 1/ Prevalence of disease

As the prevalence of a disease increases in a population, NPV decreases for the screening test. Therefore, since the prevalence of an infectious disease in a community has been reduced by 90%, its PPV will reduce and its NPV will increase.