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Biotech / Medical : Ciphergen Biosystems(CIPH):

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To: tuck who wrote (64)	9/9/2002 8:26:46 PM
From: tuck	Read Replies (1) of 510

Of course, sensitivity and specificity are of diagnostic tests important, but one must put them in context with the population, and the positive predictive value of a test. An excellent -- and short -- primer on that issue can be found here: musc.edu In the papers concerning multiple biomarkers, subjects already had the disease or were confirmed not to have it by other means. The prevalence of the disease in the population has a significant effect on the predictive value of diagnostic tests. In these studies, the prevalence is very high -- around 60% for the breast cancer tests, which yielded positive predictive values in the 80% to 90% range. In the population at large, in women over 25, the prevalence is roughly 5%. The positive predictive value is about 20%, same as mammography, until you get to women over 50 where the prevalence is higher (mammography is also better there). This is true of any test, and it is why confirmatory testing with a gold standard -- usually much more expensive -- is then done on all positives. Cheers, Tuck

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