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Disease prevalence and predictive value of a test

The proportion of correct and incorrect classifications of healthy subjects and patients is co-determined by disease prevalence in the study population. On that account the predictive value of a test in principle differs strongly in general practice, in occupational medicine and in an outpatient or clinical population. This is exemplified in the following examples, in which the sensitivity and specificity are the same, and which relate to two populations comprising 1800 subjects.

  Population A Population B
  Test result Test result
Clinically + - Total + - Total
Sick 240 30 270 16 2 18
Not sick 60 1470 1530 71 1711 1782
Totals 300 1500 1800 87 1713 1800

  Population A Population B
Prevalence 15% 1%
Sensitivity 89% 89%
Specificity 96% 96%
+ Predictive value 80% 18.4%
- Predictive value 89% 99.9%
Efficiency 95% 95.9%

The figure below illustrates how the disease prevalence in the study population affects the above indices.

False positive and false negative test results

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