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 ?
Its sensitivity has increased
Its positive predictive value has increased
Its negative predictive value has increased
The test's characteristics have not 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 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.