Screening for chlamydia and gonorrhea in primary care in populations with low prevalence.

I read the new recommendations on primary care screening for chlamydia and gonorrhea with interest, but I wanted to highlight an issue not addressed in the article.1

Although most nucleic acid amplification tests used for diagnosis of chlamydia and gonorrhea in Canada have excellent clinical sensitivity and specificity (> 95%),2 they may still generate false-positive results when used in a population with low prevalence of these diseases. Consider the example of female vaginal swabs taken for chlamydia testing in a population with a 1.5% prevalence of chlamydia (within the estimate indicated in the screening recommendations). Using a test with 97.2% sensitivity and 98.5% specificity,3 the positive and negative predictive values are 49.7% and 99.9%, respectively. That means the chance that a positive result is a false positive is greater than 50%.

The concern with false positives drops as prevalence increases; the same test in a population with 10% prevalence of chlamydia has a positive predictive value of 87.8%, so only roughly 1 in 10 positive results would be a false positive. In other words, test performance is better in higher-risk populations.

Widespread testing for chlamydia and gonorrhea can help to reduce disease transmission and complications. However, health care providers should be aware that with expanded screening in a low-prevalence population, they will see false-positive results, which may cause anxiety for patients and their partners and could lead to unnecessary interventions. Repeat testing should be considered if a false-positive result is suspected based on a patient’s individual history; a subsequent negative result is reassuring, given the high negative predictive value of nucleic acid amplification tests in this context.