Letters

Sensitivity and specificity and their confidence intervals cannot exceed 100%

BMJ 1999; 318 doi: http://dx.doi.org/10.1136/bmj.318.7177.193b (Published 16 January 1999) Cite this as: BMJ 1999;318:193
  1. Jonathan J Deeks (J.Deeks@icrf.icnet.uk), Medical statistician,
  2. Douglas G Altman, Director.
  1. ICRF/NHS Centre for Statistics in Medicine, Institute of Health Sciences, Oxford OX3 7LF

    EDITOR—Stell and Gransden investigated the diagnostic accuracy of liquid media and direct culture of aspirated fluid as tests of septic bursitis.1 They reported that culture in liquid media had a sensitivity of 100% (95% confidence interval 92% to 108%) and a specificity of 89% (74% to 104%).

    As sensitivity and specificity cannot exceed 100%, neither should their confidence intervals. Such impossible results arise when the standard large sample method for calculating confidence intervals for proportions is …

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