Abstract

In the previous two parts of educational manuscript series in Emergency, we explained some screening characteris-tics of diagnostic tests including accuracy, sensitivity, specificity, and positive and negative predictive values (1, 2). In the 3rd part we aimed to explain positive and negative likelihood ratio (LR) as one of the most reliable performance measures of a diagnostic test (3). To better understand this characteristic of a test, it is first neces-sary to fully understand the concept of sensitivity and specificity. So we strongly advise you to review the 1st part of this series again (1, 2). In short, the likelihood ratios are about the percentage of people with and without a disease but having the same test result (4). The preva-lence of a disease can directly influence screening charac-teristics of a diagnostic test, especially its sensitivity and specificity. Trying to eliminate this effect, LR was devel-oped. Pre-test probability of a disease multiplied by posi-tive or negative LR can estimate post-test probability.

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