The positive predictive value can be of more use to GPs than the likelihood ratio
The study by Steurer and colleagues provides further evidence of the difficulty that clinicians experience in applying information about the sensitivity and specificity of a diagnostic test. I support their call for a different way of expressing the reliability of a test but I disagree with their conclusion that, in their study, “most general practitioners recognised the correct definitions for sensitivity and positive predictive value but did not apply them correctly”. In the vignette, the GPs were not given the positive predictive value (PPV), and so did not have a chance to apply it. Had they done so, I suspect that they would have performed better than they did when given the positive likelihood ratio.
The group of GPs who were given the information that the positive likelihood ratio was two did better than the other groups, but they still did not do that well. Their estimates of the chance of cancer ranged from an attributed likelihood ratio of less than 2 to one of almost 30. Had they been given the PPV of the test for the diagnosis of carcinoma in a woman of 64 with abnormal uterine bleeding, it is hard to see that many would have failed to give the correct answer to the question: “what is the probability that this woman has endometrial carcinoma” since, by definition, that is the PPV.
The problem with the PPV for any one test, of course, is that it will vary according to the prevalence, which is in turn dependant, in this case, on the age of the patient, her symptoms, and the setting in which she is seen. That can be coped with by giving two or more PPVs, each one related to women with abnormal uterine bleeding seen in primary care at different ages, say 25 and 65. From these the GP can extrapolate to apply the PPV to the patient in question. GPs consult without having access to information about pre-test probabilities and usually without a nomogram from which to calculate post-test probabilities, if given the likelihood ratio. A small range of PPVs for each test would be of more use to them in the real world of primary care.
Competing interests: No competing interests