The concepts of sensitivity, specificity and predictive value and how they can be applied to clinical testing are discussed in this third article in a series of four outlining a framework to interpret the results of pathology laboratory tests.
A correction for this article was published in the November 2015 issue of Medicine Today. The full text PDF for this article (see link above) has been corrected.
Tests may be performed because they provide a diagnosis in a patient with specific symptoms (diagnostic tests) or because they give information about the likelihood of an asymptomatic disease being present or likely to develop in a person without specific symptoms (screening tests). Many people believe that a positive laboratory test result is always positive for the disease and a negative laboratory test result excludes it. But sometimes a positive result is a false positive and a negative result is a false negative. The predictive value of a positive or negative result depends on the characteristics of the test itself (its sensitivity and specificity) and the likelihood of disease before the test is performed. In many cases of screening for a problem that has a low probability of being present, the sensitivity and specificity of the screening test are such that a positive result is more likely to be a false positive than an indicator of the presence of the disease.
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