The clinical usefulness of a diagnostic test is determined by the extent to which it helps to modify the pretest probability of occurrence of a certain diagnosis. For this purpose, the calculation and application of likelihood ratios (LR) is a very useful tool. Every possible result of a test has a LR, which allows for an estimation of the magnitude by which the pretest probability increases or decreases, thereby helping the physician rule out, confirm, or continue investigating a diagnosis with new tests. In the first part of this article the properties of a diagnostic test (validity, reproducibility and safety) were analyzed. This second part illustrates the theoretical background underlying the concept of LR, explains the methodology for calculating them and describes their clinical application.