Sensitivity and specificity, which are integral to validity, are important considerations in decision making in medicine, particularly in choosing appropriate investigations for disease diagnoses. The method of calculation of sensitivity and specificity, and predictive values is summarised in Figure 13.1 . The sensitivity of a test depends on the proportion of people with the characteristic (disease) in whom the test is positive (that is, percentage positive with disease). The ultimate sensitive test is one that detects all true positive cases.
The specificity of a test depends on the proportion of people without the characteristic (disease) in whom the test is negative (that is, percentage negative of healthy people). The ultimate specific test is one that detects all the truly negative (disease-free) cases. A clinical example of sensitivity and specificity is presented in Table 13.1 .
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