16.4: Sensitivity and Specificity of Serologic Testing

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Sensitivity measures the "true-positive" rate of the serologic test, that is, how often a test correctly generates a positive result for people who have the disease being tested. To give an example, a test having a 95% sensitivity will correctly give a positive result for 95% of the people who have that disease, but will give a negative result, or false-negative, for 5% of the people who have the disease and should have tested positive. So, a high-sensitivity serologic test will correctly identify almost everyone who does have the disease and will generate few false-negative results.

Specificity measures the "true-negative" rate of the serologic test, that is, how often a test correctly generates a result for people who do not have the disease being tested. As an example, a test having a 95% specificity will correctly give a negative result for 95% of people who do not have the disease, but will return a positive result, or false-positive, for 5% of the people who don't have the disease and should have tested negative. So, a high-specificity serologic test will correctly rule out almost everyone who does not have the disease and will generate few false-positive results.

However, no serologic tests are 100% accurate. When considering test accuracy, the rate of infection also needs to be considered. In a region with a low disease prevalence, the risk of false positive results by serologic testing is higher, even with excellent specificity. However, in a region with a high disease prevalence, the risk of false negative results will be higher, even with excellent sensitivity.

Caution

Most of these tests look for the presence of antibodies (often IgM made early during an infection, as well as IgG made later during an infection and for a longer duration) against the N (nuclear) antigen of SARS-CoV-2. There is a greater problem with false-positive and false-negative results compared to the tests that detect SARS-CoV-2 RNA but test results can be determined much more quickly. Antibody tests are not currently recommend as the sole basis for diagnosis of acute COVID-19 infection.

Further Information

Medscape articles on infections associated with organisms/diseases mentioned in this lab exercise. Registration to access this website is free.

Contributors and Attributions

Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS)