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General Practice

# Statistics Notes: Diagnostic tests 2: predictive values

BMJ 1994; 309 (Published 09 July 1994) Cite this as: BMJ 1994;309:102
1. Douglas G Altman, head Department of Public Health Sciences, St George's Hospital Medical School, London SW17 0REa,
2. J Martin Bland, reader in medical statisticsa
1. a Medical Statistics Laboratory, Imperial Cancer Research Fund, London WC2A 3PX

The whole point of a diagnostic test is to use it to make a diagnosis, so we need to know the probability that the test will give the correct diagnosis. The sensitivity and specificity1 do not give us this information. Instead we must approach the data from the direction of the test results, using predictive values.

Positive predictive value is the proportion of patients with positive test results who are correctly diagnosed.

Negative predictive value is the proportion of patients with negative test results who are correctly diagnosed.

Using the same data as in the previous note,1 we know that 231 of 263 patients with abnormal liver scans had abnormal pathology, giving the proportion of correct diagnoses as 231/263 = 0.88. …

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