Nov 03, 2015
Terminology 101: Screening tests and specificity
Specificity: A calculation that determines the proportion of patients who don’t have the disease who are correctly classified as such by the screening test
Source: Webb, P., & Bain, C. (2011). Essential Epidemiology: An Introduction for Students and Health Professionals. (2nd ed.). Cambridge, UK: Cambridge University Press
Like sensitivity, specificity is an important measure used to judge the performance of a screening test against that of a gold-standard test. It refers to the ability of the test to correctly identify people who don’t have the disease. A specificity score answers this question: what proportion of individuals without the disease correctly receive a negative test result? To calculate the score, one needs to know the number of true negative (TN) and false positive (FP) results associated with the test. For individuals who do not have the disease, negative test results are referred to as TN, while positive test results are referred to as FP.
To illustrate how specificity is calculated, let us consider colon cancer screening. The fecal occult blood test (FOBT) is an inexpensive alternative to the more costly and invasive colonoscopy (the gold standard). Say that we screened 100 adults for colon cancer using FOBT and find that 75 received negative results and 25 received positive results. The 25 people who received positive results are then subjected to additional screening using colonoscopy, which shows that five of them are actually disease free. Assuming that the 75 individuals with negative results are indeed disease free, the TN count associated with FOBT is 75 and the FP count is five. The specificity score is calculated by dividing the TN count by the sum of the TN and FP counts (TN/[TN+FP]). In our example, the score is about 94 per cent, which is impressive for such an inexpensive test.
The lower the number of FP cases, the higher the specificity score of the test. The minimum specificity value deemed to be acceptable depends on the nature of the health condition in question and the primary goal of the screening. If the disease is life-threatening or highly contagious, we will probably be willing to tolerate a larger number of FPs, and thus a lower specificity score, because we want to avoid missing cases of the disease. Individuals for whom the screening test produces a positive result can always go on to have more rigorous testing. However, if the purpose of the screening is to determine whether we can rule out the disease, it is important that the test have a high specificity score so that we can be confident that those who test negative do not have the disease. An example of such a scenario would be the use of the nitrate dipstick test to rule out urinary tract infections in hospitalized patients.
Sensitivity (addressed in the October 2015 issue) and specificity are often inversely related — as the value of one goes up, the value of the other goes down. An ideal screening test will have high sensitivity and specificity scores.
NurseONE.ca resources on this topic
- Fitzpatrick, J. J., & Kazer, M. W. (Eds.). (2011). Encyclopedia of Nursing Research (3rd ed.).
- Maltby, J., Williams, G., McGarry, J., & Day, L. (2010). Research Methods for Nursing and Healthcare.
- Supino, P. G., & Borer, J. S. (Eds.). (2012). Principles of Research Methodology: A Guide for Clinical Investigators.