Pre-test probability of disease can be compared with the estimated later probability of disease using the information provided by a diagnostic test. The difference between the previous probability and the later probability is an effective way to analyze the efficiency of a diagnostic method. Post-test probability is calcualted using likelihood ratios of the method and pre-test probability or prevalence of the disease.
In order to transform the information given by these parameters, Bayes´ theorem is used as follows:
Pretest-probability = p1 (population prevalence)
Pretest odds = p1/(1-p1)
Post-test odds = p2 = pretest odds x likelihood ratio
Post-test probability = p2/(1+p2)