all those healthy (b+d).
Although sensitivity and specificity are of interest to
public-health policymakers, they are of little use to the
clinician. Stated alternatively, sensitivity and specificity
(population measures) look backward (at results gathered
over time).8 Clinicians have to interpret test results to
those tested. Thus, what clinicians need to know are the
predictive values of the test (individual measures, which
look forward). To consider predictive values, one needs to
shift the orientation in figure 1 by 90 degrees: predictive
values work horizontally (rows), not vertically (columns).
In the top row are all those with a positive test, but only
those in cell a are sick. Thus, the predictive value positive
is a/(a+b). The “odds of being affected given a positive
result (OAPR)” is the ratio of true positives to false
positives, or a to b.10 For example, in figure 1, the OAPR
is 75/5, or 17/1. This corresponds to a positive predictive
value of 89%. Advocates of use of the OAPR note that
these odds better describe test effectiveness than do
probabilities (predictive values). In the bottom row of
figure 1 are those with negative tests, but only those in cell
d are free of disease. Hence, the predictive value negative
is d/(c+d).