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Editor's Note
June 2014

Ensuring Correct Interpretation of Diagnostic Test Results

JAMA Intern Med. 2014;174(6):993. doi:10.1001/jamainternmed.2014.165

Today, physicians order a wide array of diagnostic laboratory and imaging tests for their patients, including genetic evaluations. To make sense of the growing number of diagnostic testing opportunities, one might expect that physicians, in turn, have grown in their ability to accurately interpret test results. The Research Letter by Manrai and colleagues1 finds this not to be the case. They replicated a classic study and found that only 23% of physicians and physicians-in-training correctly answered a single question testing their interpretation of a diagnostic test result. While this study was limited to a convenience sample from a single academic teaching hospital, it is not too far out on a limb to suggest that today’s physicians need to be better prepared to interpret diagnostic test results, including stronger training in statistics and clinical epidemiology.

In addition, this study reminds us that disease prevalence matters for testing, as does test accuracy (both sensitivity and specificity). However, these important pieces of information are often lacking at the bedside when we make a decision to order a test. Generally, prevalence is considered as only “rare” or “common.” And how often are physicians aware of diagnostic tests’ sensitivity and specificity? We need resources that make this information more easily and readily available.

In the meantime, before ordering any test, we must ask ourselves if it is even necessary. Assuming there are efficacious treatments for the disease being tested, what are our thresholds for “ruling out” disease on the low end and “ruling in” disease on the high end of probability, and then, what is the pretest probability of the disease? If your pretest probability falls between those thresholds, is the test accurate enough that a positive or negative test finding will result in a posttest probability that crosses these thresholds? If the test result is not going to change your clinical management, there is no reason for the patient to undergo testing in the first place.

The persistent inability of physicians to reliably manage this cognitive exercise implies that our educational programs need to do a better job at teaching numeracy skills. Because imprecise diagnostic decision making is leading to excessive testing, patient harm, and excessively costly care, we must raise the bar and master these cognitive skills.

Manrai  AK, Bhatia  G, Strymish  J, Kohane  IS, Jain  SH.  Medicine’s uncomfortable relationship with math: calculating positive predictive value [published online April 21, 2014].  JAMA Intern Med. doi:10.1001/jamainternmed.2014.1059.Google Scholar
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    1 Comment for this article
    The culture of \"Rule Out\"
    J. David Spence | Robarts Research Institute, Western University
    The problem, as intimated by Dr. Ross, is the culture of \"rule out\". I well remember my experiences with the phenomenon of \"the workup\" as a visiting medical student at Jackson Memorial Hospital in Miami, and then as a Fellow in Clinical Pharmacology at Moffitt Hospital at UCSF. I was amazed at the concept of \"the workup\", which was an exhaustive list of investigations that would be ordered in all patients presenting with a given problem. The question from the attending or chief resident would be \"What is the workup for (condition X)?\", and the \"correct\" answer from the student or junior resident would be a long list of investigations, most of which were not indicated. In the more British system in which I was educated in Canada, and which I hope persists, at least to some degree, a diagnosis is approached by \"a thorough history and physical examination, and judicious use of laboratory investigations\". In that context, the pretest probability is much higher than when a shotgun blast of investigations is thrown at the patient, hoping that one might hit the tiny target