LIKELIHOOD RATIOS - GETTING DIAGNOSTIC TESTING INTO PERSPECTIVE

In modern medicine, sophisticated laboratory tests and imaging studies are often emphasized at the expense of history and physical examinati on, rather than complementing clinical assessment. Ancillary testing o ften fails to advance the diagnostic process, and increases patient ri sk and the expense of medical care. The relative value of clinical eva luation and technological methods is rarely considered, and the power of the clinical evaluation is therefore underestimated. The likelihood ratio (LR) is a semiquantitative measure of the performance of diagno stic tests which indicates how much a diagnostic procedure modifies th e probability of disease, and is calculated from the sensitivity and s pecificity of the test (or directly from the change in probability ass ociated with the test result). We review the performance of frequently -used tests by their LRs, and compare them to the power of clinical as sessment, with clinical cases to illustrate the application of LRs in the diagnostic process. The discriminative power of clinical assessmen t and ancillary tests is often similar, and the combination of the two greatly increases accuracy in the diagnostic process. Clinical assess ment is indeed frequently more informative than current technical moda lities. LRs assist in putting the value of testing in proper perspecti ve. practice in evaluating pre-test probabilities of disease and in th e application of LRs should be enhanced in medical training.