Endgames

Statistical Question

Screening tests II

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

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  1. Philip Sedgwick, senior lecturer in medical statistics,
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London SW17 0RE

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

    A new rapid urine test was evaluated as a screening tool for Chlamydia trachomatis infection in men.2 The test was compared with the gold standard diagnostic test for Chlamydia infection—polymerase chain reaction (PCR) assay. The rapid urine screening test was reported to have a positive predictive value of 84.1%, negative predictive value of 98.3%, and a likelihood ratio for a positive result of 53.5.

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

    • a) Definition of positive predictive value: out of all of the men with a “positive” result on the rapid urine screening test, 84.1% had a diagnosis of Chlamydia infection on the …

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