Due to an effective prevention program, the pre valence 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?
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 á 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
Its sensitivity has increased
Its positive predictive value has increased
The test’s characteristics have not changed
Its negative predictive value has increased