The Complex Interpretation and Management of Zika Virus Test Results

Kenneth W. Lin, MD, MPH; John D. Kraemer, JD, MPH; Rachael Piltch-Loeb, MSPH; Michael A. Stoto, PhD


J Am Board Fam Med. 2018;31(6):924-930. 

In This Article

Sidebar: Definitions of Test Characteristics

Screening tests are often described in terms of sensitivity and specificity. Sensitivity describes the test's ability to detect cases where they actually exist. Sensitivity is defined as probability that test is positive, given that condition is present. Specificity describes the likelihood that the test will avoid false positives, that is to be negative when the condition is not present. Specificity is defined as the probability that, if the condition is not present, the test will be negative.

Sensitivity and specificity describe how well a test performs given the "truth," that is, if the person being tested has or does not have the condition of interest. In practice, however, one knows the test result and wants to know whether the patient has the condition. Epidemiologists use PPV and negative predictive value (NPV) to describe how well a test distinguishes between individuals with and without the condition. PPV is defined as the probability that the condition is present, given that the test is positive; NPV is the probability the condition is not present, given that the test is negative.

Unlike sensitivity and specificity, the positive and negative predictive values depend on the prevalence of the condition. For example, if a test has 80% sensitivity and 90% specificity and the prevalence of the condition is 10%, the PPV is only 47.1%. This is because in a group of 1000 individuals, 100 of whom have the condition, there will be 90 false positives (10% of 900) and only 80 true positives (80% of 100). When the prevalence is 1%, only 8 of 107 positive tests are for people who actually have the condition, so the PPV is only 7.5%. On the other hand, nearly all the individuals with negative test values do not in fact have the condition, so the NPV is 891/893 (99.8%).