What tests are used to diagnose Sexually Transmitted Infections (STIs)?

Diagnostic Testing for Sexually Transmitted Infections (STIs)

Comprehensive STI testing should include nucleic acid amplification tests (NAATs) for chlamydia, gonorrhea, and trichomoniasis, serologic testing for syphilis and HIV, and targeted testing based on exposure sites and risk factors. 1

Core STI Testing Components

Standard Testing for All Sexually Active Patients

• Syphilis serologic testing: Most laboratories use a reverse screening algorithm with treponemal-specific test first (EIA/chemiluminescence immunoassay) followed by nontreponemal test (RPR) to confirm 1
• Gonorrhea testing:
  • Men: Urine NAAT
  • Women: Vaginal swab (preferred), cervical swab, or urine NAAT 1
• Chlamydia testing:
  • Men: Urine NAAT
  • Women: Vaginal swab (preferred), cervical swab, or urine NAAT (especially important if sexually active and aged 25 years or younger) 1
• HIV screening: Recommended for all sexually active patients aged 13-64 seeking evaluation for STIs 1

Additional Testing Based on Risk Factors and Symptoms

• Trichomoniasis testing: Vaginal swab NAAT (preferred) or culture/rapid antigen detection test for women 1
• HSV testing: For genital lesions, use viral culture, direct fluorescent antibody (DFA) and/or NAATs; type-specific (glycoprotein G-based) serology may be considered 1
• Hepatitis B and C screening: Particularly important for MSM and high-risk individuals 1
• HPV testing: Co-testing with high-risk HPV and cytology increases detection of cervical cancer compared to cytology alone; particularly important for women >30 years of age 1

Site-Specific Testing for High-Risk Groups

• For patients reporting receptive anal sex:

  • Rectal culture or NAAT for gonorrhea (if performed at laboratory with validation)
  • Rectal NAAT for chlamydia (if performed at laboratory with validation) 1

• For patients reporting receptive oral sex:

  • Pharyngeal culture or NAAT for gonorrhea (if performed at laboratory with validation) 1

Testing Frequency and Special Populations

• Annual screening: Recommended for all sexually active patients 1

• More frequent screening (every 3-6 months) recommended for patients with:

  • Multiple partners
  • Anonymous partners
  • History of STIs
  • Substance use, especially methamphetamine
  • Unprotected sex outside monogamous relationships
  • Sex work or partners who engage in sex work 1

• Pregnant patients: Screen for HIV at first prenatal visit and third trimester; screen for group B streptococcus at 35-37 weeks using both vaginal and rectal swabs 1

• Men who have sex with men (MSM): Should have extragenital sites evaluated (rectal, oropharyngeal) for gonorrhea and chlamydia 1

Emerging Diagnostic Considerations

• Mycoplasma genitalium: Emerging pathogen causing nongonococcal urethritis in males and cervicitis/PID in females 1
• Multiplex PCR testing: Newer platforms allow simultaneous testing for multiple STIs, improving efficiency and detection rates 2
• Point-of-care testing: Rapid diagnostic tests are becoming available for syphilis, chlamydia, gonorrhea, and trichomoniasis, allowing for immediate treatment 3

Important Clinical Considerations

• 25-40% of genital infections may not have an identifiable cause despite diagnostic testing 1
• Partner testing and treatment is essential to prevent reinfection, especially for pregnant or HIV-positive patients 1
• Positive tests for chlamydia, gonorrhea, syphilis, chancroid, and HIV require public health reporting in most jurisdictions 1
• For genital lesions, all patients should be evaluated with serologic testing for syphilis, diagnostic tests for herpes, and HIV screening 1

Common Pitfalls to Avoid

• Inadequate specimen collection: Ensure proper collection technique and appropriate transport conditions to maintain specimen integrity 1
• Overlooking extragenital testing: Rectal and pharyngeal infections are common in high-risk populations and may be missed with urogenital-only testing 1
• Failing to retest after treatment: Follow-up testing is essential to confirm cure and detect reinfection 1
• Neglecting partner notification: Many reinfections occur due to untreated partners 1
• Relying on symptoms alone: Most STIs are asymptomatic, making screening essential for detection 1