Test Performance based on prevalence of disease

Both PPV and NPV vary by the prevalence of disease (PPV is directly proportional to prevalence, and NPV is inversely proportional to prevalence). Therefore, the performance of the test will vary by prevalence of COVID-19 in the population. The PPV and NPV for a range of prevalence values is presented in the table below:

Note: False –ve refers to the cell value of cell ‘c’. This is dependent on the sensitivity of the test. Here (columns D and F), the value changes not because sensitivity is changing, but because the disease prevalence is changing. Columns D and F show the number of False –ve test results per 1,000,000 tests.

False +ve refers to the cell value of cell ‘b’. This is dependent on the specificity of the test. Here (column H), the value changes due to change in disease prevalence, not specificity of the test. Column H shows the number of False +ve test results per 1,000,000 tests.