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- Question 1: Is this test potentially relevant to my practice? (Greenhalgh, 1997)

Sackett and colleagues call this the utility of the test Even if this test were 100% valid, accurate, and reliable, would it help me? Would it identify a treatable disorder? If so, would I use it in preference to the test I use now? Could I (or my patients or the taxpayer) afford it? Would my patients consent to it? Would it change the probabilities for competing diagnoses sufficiently for me to alter my treatment plan?

- Question 2: Has the test been compared with a true gold standard? (Greenhalgh, 1997)

You need to ask, firstly, whether the test has been compared with anything at all. Assuming that a "gold standard" test has been used, you should verify that it merits the description, perhaps by using the questions listed in question 1. For many conditions, there is no gold standard diagnostic test. Unsurprisingly, these tend to be the conditions for which new tests are most actively sought. Hence, the authors of such papers may need to develop and justify a combination of criteria against which the new test is to be assessed. One specific point to check is that the test being validated in the paper is not being used to define the gold standard.

- Question 3: Did this validation study include an appropriate spectrum of subjects? (Greenhalgh, 1997)

Although few investigators would be naive enough to select only, say, healthy male medical students for their validation study, only 27% of published studies explicitly define the spectrum of subjects tested in terms of age, sex, symptoms or disease severity, and specific eligibility criteria. Importantly, the test should be verified on a population which includes mild and severe disease, treated and untreated subjects, and those with different but commonly confused conditions. Although the sensitivity and specificity of a test are virtually constant whatever the prevalence of the condition, the positive and negative predictive values depend crucially on prevalence. This is why general practitioners are skeptical of the utility of tests developed exclusively in a secondary care population, and why a good diagnostic test is not necessarily a good screening test.

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