- Page navigation anchor for RE: Recommendation on screening for chlamydia and gonorrhea in primary careRE: Recommendation on screening for chlamydia and gonorrhea in primary care
I read with interest the new recommendations on primary care screening for chlamydia and gonorrhea but I wanted to highlight one issue not addressed in the article (1).
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While most NAATs (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 low-prevalence population. Consider the example of female vaginal swabs taken for chlamydia testing in a population with a chlamydia prevalence of 1.5% (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 (PPV and NPV) are 49.7% and 99.9% respectively. That means the chance a positive result is a false-positive is >50%.
The concern with false positives drops as prevalence increases – the same test in a population with 10% chlamydia prevalence has a PPV 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.
There is value in widespread testing for chlamydia and gonorrhea 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 unnecessar...Competing Interests: None declared.References
- Ainsley Moore, Gregory Traversy, Donna L. Reynolds, et al. Recommendation on screening for chlamydia and gonorrhea in primary care for individuals not known to be at high risk. CMAJ 2021;193:E549-E559.
- Max Chernesky, Dan Jang, Jodi Gilchrist, et al. Head-to-Head Comparison of Second-Generation Nucleic Acid Amplification Tests for Detection of Chlamydia trachomatis and Neisseria gonorrhoeae on Urine Samples from Female Subjects and Self-Collected Vaginal
- Aptima Combo 2 Assay (Panther System) product insert, Hologic, 502446 Rev 003, Accessed online June 15, 2021 at file:///H:/Diana's%20documents/Genital%20bench/Aptima%20product%20inserts/Aptima%20Combo%202%20assay%20product%20insert.pdf
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