How to Improve STI Test Accuracy – Swab or Urine?

June 12, 2023

Written by Aaron Lacy and Mitchell Cross

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Vaginal swabs are more accurate than urine samples in diagnosing Chlamydia trachomatis, Neisseria gonorrhoeae, and Trichomonas vaginalis. These results support CDC recommendations that vaginal swabs are the preferred sample modality when testing for STIs in women.

Why does this matter?
It is estimated that hundreds of millions of curable STIs are spread each year. Women bear a disproportionate burden of these infections. Accurate diagnosis is essential for timely treatment and control of these infections.

Urine for some bad news
Despite CDC guidelines stating better sensitivity of vaginal swabs when testing for chlamydia, gonorrhea, and trichomonas, the most commonly sent tests are urine studies. The researchers conducted a systematic search of multiple databases from 1995 to 2021 to identify relevant studies that met specific inclusion criteria. A total of 28 eligible articles were included. Pooled sensitivity estimates were calculated for each pathogen for both vaginal swabs and urine specimens, along with corresponding 95% confidence intervals (CIs) and odds ratios (ORs) to compare the performance of the two sample types. When comparing vaginal and urine nucleic acid amplification tests to a reference standard (alternative NAAT, culture, microscopy, etc), the authors found the following pooled sensitivity results:

Chlamydia trachomatis

• 20 studies, 30 comparisons
• Sensitivity: Vaginal swabs 94.1%, urine specimens 86.9% (p < 0.001)

Neisseria gonorrhoeae

• 10 studies, 16 comparisons
• Sensitivity: Vaginal swabs 96.5%, urine specimens 90.7% (p < 0.001)

Trichomonas vaginalis

• 9 studies, 9 comparisons
• Sensitivity: Vaginal swabs 98.0%, urine specimens 95.1% (p < 0.001)

Key takeaways

The study was limited to commercially available assays and studies published in English. The authors estimate that by sending urine samples alone, up to 400,000 STIs per year could be missed. This is of particular concern when screening asymptomatic women who may have a lower organism load. The CDC recommends cervical or vaginal swabs, not urine. If you are ordering urine swabs on patients, make sure it’s not because you don’t want to do a swab. Other issues with urine testing are that when looking for STIs, a “first-catch” urine sample should be done; often, when providing a sample, it is a “clean-catch” sample that is provided.

A risk-benefit discussion should be had with patients who prefer providing a urine sample over vaginal swab. Consider having your patient do a self-collected vaginal swab to help with both patient flow and comfort. Obtaining a vaginal sample is concordant with both CDC guidelines and recent evidence – do the right thing for your patient.

Source
Vaginal Swab vs Urine for Detection of Chlamydia trachomatis, Neisseria gonorrhoeae, and Trichomonas vaginalis: A Meta-Analysis. Ann Fam Med. 2023 Mar-Apr;21(2):172-179. doi: 10.1370/afm.2942.

Peer Reviewed by Ketan Patel and Clay Smith