Epidemiology 3: likelihood ratios

The Journal : Current Issue: "Likelihood ratios can refine clinical diagnosis on the basis of signs and symptoms; however, they are underused for patients' care. A likelihood ratio is the percentage of ill people with a given test result divided by the percentage of well individuals with the same result. Ideally, abnormal test results should be much more typical in ill individuals than in those who are well (high likelihood ratio) and normal test results should be most frequent in well people than in sick people (low likelihood ratio). Likelihood ratios near unity have little effect on decision-making; by contrast, high or low ratios can greatly shift the clinician's estimate of the probability of disease. Likelihood ratios can be calculated not only for dichotomous (positive or negative) tests but also for tests with multiple levels of results, such as creatine kinase or ventilation-perfusion scans. When combined with an accurate clinical diagnosis, likelihood ratios from ancillary tests improve diagnostic accuracy in a synergistic manner.

Despite their usefulness in interpretation of clinical findings, laboratory tests, and imaging studies, likelihood ratios are little used. Most doctors are unfamiliar with such ratios, and few use them in practice. In a survey of 300 doctors in different specialties, only two (both internists) reported using likelihood ratios for test results.1 Since simple descriptions help clinicians to understand such ideas,2 we will try to make likelihood ratios both simple and clinically relevant.3 Our aim is to enhance clinicians' familiarity with and use of likelihood ratios."