Sensitivity: the proportion of patients with the target disorder who have a positive test result (a/[a + c]) (figure).
Specificity: the proportion of patients without the target disorder who have a negative test result (d/[b + d]) (figure).
Pretest probability (prevalence): the proportion of patients who have the target disorder, as determined before the test is carried out ([a + c]/[a + b + c + d]) (figure).
Pretest odds: the odds that the patient has the target disorder before the test is carried out (pretest probability/ [1 – pretest probability]).
Likelihood ratio (LR): the ratio of the probability of a test result among patients with the target disorder to the probability of that same test result among patients who are free of the target disorder. The LR for a positive test is calculated as sensitivity/(1 – specificity). The LR for a negative test is calculated as (1 – sensitivity)/specificity.
Post-test odds: the odds that the patient has the target disorder after the test is carried out (pretest odds x LR).
Post-test probability: the proportion of patients with that particular test result who have the target disorder post-test odds/[1 + post-test odds]).
Figure 1 Comparison of test results with a diagnostic standard
