Pretest probability is the baseline risk for having the condition of interest. In other terms, it is the prior probability of having the disease without having the test carried out. Pretest probability is another name for the prevalence of a disease in a population.

Prevalence (pretest probability) =

(TP + FN) / (TP + FN + FP + TN) =

(a + c) / (a + c + b + d)