Epidemiologic evaluation of screening for risk factors: application to genetic screening.

To assess the usefulness of screening for risk factors, we derived arithmetic relationships between screening parameters (sensitivity, specificity, and positive predictive value PPV) and risk factor frequency, disease frequency and relative risk. We evaluated these relationships in the special case of genetic markers and disease susceptibility. It can be shown that even in the face of very large relative risks, sensitivity and positive predictive value are affected by the relative magnitude of disease and genetic marker frequencies. When the genetic marker is less frequent than the disease, PPV increases with increasing relative risk but sensitivity remains low. When the genetic marker is more frequent than the disease, sensitivity increases with increasing relative risk but PPV remains low. When marker and disease frequencies are equal, both PPV and sensitivity increase with increasing relative risks, but very high relative risks (greater than 100) have to be obtained for rare diseases. Depending on the goals of the screening program, these relationships can be used to predict the relative magnitudes of false positives (low PPV) and false negatives (low sensitivity). This approach can be generalized to evaluate nongenetic risk factors in screening programs as well.