Effect of disease prevalence on predictive values

Consider a population of 2000 people and a diagnostic test that is 90 percent sensitive and specific. The positive and negative predictive values of the test vary when the prevalence of the disease is varied.
Left panel: If the prevalence of the disease is high at 50 percent, the positive and negative predictive values will be similar to the sensitivity and specificity (90 percent, respectively).
Right panel: In contrast, if the prevalence of the disease is only 1 percent, the positive predictive value decreases (8 percent) and the negative predictive value increases (99 percent).