How to read a paper: Papers that report diagnostic or screening tests

If you are new to the concept of validating diagnostic tests, the following example may help you. Ten men are awaiting trial for murder. Only three of them actually committed a murder; the seven others are innocent of any crime. A jury hears each case and finds six of the men guilty of murder. Two of the convicted are true murderers. Four men are wrongly imprisoned. One murderer walks free. PETER BROWN This information can be expressed in what is known as a two by two table (table 1). Note that the “truth” (whether or not the men really committed a murder) is expressed along the horizontal title row, whereas the jury's verdict (which may or may not reflect the truth) is expressed down the vertical row. View this table: Table 1 Two by two table showing outcome of trial for 10 men accused of murder These figures, if they are typical, reflect several features of this particular jury: These five features constitute, respectively, the sensitivity, specificity, positive predictive value, negative predictive value, and accuracy of this jury's performance. The rest of this article considers these five features applied to diagnostic (or screening) tests when compared with a “true” diagnosis or gold standard. A sixth feature—the likelihood ratio—is introduced at the end of the article. Our window cleaner told me that he had been feeling thirsty recently and had …