Likelihood Ratio (LR)« Back to Glossary Index
For a screening or diagnostic test (including clinical signs or symptoms), the LR expresses the relative likelihood that a given test would be expected in a patient with, as opposed to one without, a disorder of interest. A likelihood ratio of 1 means that the post-test probability is identical to the pre-test probability. As likelihood ratios increase above 1, the post-test probability progressively increases in relation to the pre-test probability. As likelihood ratios decrease below 1, the post-test probability progressively decreases in relation to the pre-test probability. A likelihood ratio is calculated as the proportion of target positive with a particular test result (which, with a single cut point, would be either a positive or negative result) divided by the proportion of target negative with same test result.