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EDUCATION AND DEBATE: Jonathan J Deeks and Douglas G Altman Diagnostic tests 4: likelihood ratios BMJ 2004; 329: 168-169 [Full text]

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Likelihood ratios for dynamic decision-making

Alireza Moayyeri, Akbar Soltani   (5 August 2004)

Likelihood ratios for dynamic decision-making 5 August 2004
Alireza Moayyeri, Research Fellow Endocrinology & Metabolism Research Center, Shariati hospital, Tehran 14114, Iran, Akbar Soltani Send response to journal: Re: Likelihood ratios for dynamic decision-making	Two recently published articles in BMJ, one in endorsement of likelihood ratios (LRs) (1) and the other in censure of SnNOut and SpPIn properties of the diagnostic tests (2), have truly emphasized on the shortcomings accompanying sensitivity and specificity and the need for utilization of LRs. We believe, however, that the most crucial characteristic of LRs, which have not been accentuated properly, is their dynamicity and applicability in clinical settings. In a static circumstance, we need tests just to rule in or rule out a diagnosis. The most renowned directive for this purpose is use of signs and symptoms with high sensitivity or specificity to SnNOut or SpPIn. Nevertheless, getting a positive answer to a sensitive test, for instance, has no effect on decision-making process and discontinues the progression. Moreover, assuming that a diagnosis can be ruled in or ruled out with confidence, when in reality it cannot, could have serious consequences for patients (2). In a dynamic state, however, we are dealing with probabilities and thresholds that describe our understanding of the disease. In this approach, we can make the diagnosis or start the therapy just after surpassing the diagnostic or treatment threshold. In clinical circumstances, typically full of uncertainty (3), we need tests to change the degree of our uncertainty in order to surpass our estimate of probability of a disease around a predefined threshold. LRs are dynamic measures that have this strength. In clinical practice, using a nomogram or even simple consideration of the significance of LR of a test, we can re-estimate the probability of a disease. The desire to completely rule in or rule out a diagnosis is almost a plague induced by mathematical (intuitive) thinking in clinical medicine. Contrary to this viewpoint, evidence-based diagnostic approach appreciates existence, and physicians’ inability to get rid of, uncertainty in clinical medicine. LRs are the most dynamic and the most applicable tools for refining probability in clinical settings. 1) Deeks JJ, Altman DG. Diagnostic tests 4: likelihood ratios. BMJ 2004;329:168-9 2) Pewsner D, Battaglia M, Minder C, Marx A, Bucher HC, Egger M. Ruling a diagnosis in or out with "SpPIn" and "SnNOut": a note of caution. BMJ 2004;329:209-13 3) Hunink M, Glasziou P. Decision making in health and medicine: integrating evidence and values. Cambridge University Press, 2001. Competing interests: None declared