Screening tests II

doi:10.1136/bmj.b4030

Published 7 October 2009, doi:10.1136/bmj.b4030
Cite this as: BMJ 2009;339:b4030

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Statistical Question

Screening tests II

Philip Sedgwick, senior lecturer in medical statistics,

¹ Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London SW17 0RE

The first 150 words of the full text of this article appear below.

This question follows on from last week’s question onindices used to describe the performance of a screening test.¹These indices help when assessing new screening tools for usein clinical practice.

A new rapid urine test was evaluated as a screening tool forChlamydia trachomatis infection in men.² The test was comparedwith the gold standard diagnostic test for Chlamydia infection—polymerasechain reaction (PCR) assay. The rapid urine screening test wasreported to have a positive predictive value of 84.1%, negativepredictive value of 98.3%, and a likelihood ratio for a positiveresult of 53.5.

Which of the following statements, if any, are true?

a) Definition of positive predictive value: out of all of themenwith a "positive" result on the rapid urine screening test,84.1%had a diagnosis of Chlamydia infection on the PCR assay.

b)Definition of negative predictive value: out of all of themen. . . [Full text of this article]

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