Expressing Screening Test Accuracy

When deciding whether an assessment is a "good screening test," the accuracy of the test and the prevalence of the disease in the population to be screened are important factors. The accuracy of a test is its ability to measure the actual value of the quantity being measured. Sensitivity and specificity are two measures used to express the accuracy of a screening or diagnostic test. Sensitivity is defined as the proportion of people with the target disorder who have a positive test result. Specificity is the proportion of people without the target disorder who have a negative test result. Sensitivity and specificity do not vary in relation to the prevalence of the condition being tested.

Positive and negative predictive values take into account the accuracy of the screening test and the prevalence of the disease, to express the likelihood that a test result is a true result rather than a false-positive or false-negative result. The positive predictive value is the proportion of people with a positive test result who have the target disorder. The negative predictive value is the proportion of people with a negative test result who are free of the target disorder. The positive predictive value is higher and the negative predictive value lower when a test is used in a population with a higher prevalence. Clinicians need to remember that in a population with a low prevalence of a specific disease, a positive test result is likely to be a false-positive result, even for a test with a high specificity.

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