What blood tests are recommended for diagnosing sexually transmitted diseases?

Blood Tests for Diagnosing Sexually Transmitted Diseases

Blood tests are the primary diagnostic method for several STDs, with serologic testing for syphilis and HIV being the most established and widely recommended screening approaches.

Syphilis Blood Tests

Syphilis testing is primarily conducted through blood tests using one of two algorithms:

Traditional Algorithm

• Nontreponemal tests first (screening):
  • Rapid Plasma Reagin (RPR)
  • Venereal Disease Research Laboratory (VDRL)
• Followed by treponemal tests (confirmation):
  • T. pallidum particle agglutination (TP-PA)
  • Fluorescent treponemal antibody absorption (FTA-ABS)
  • Enzyme immunoassay (EIA)
  • Chemiluminescent immunoassay (CIA) 1

Reverse Sequence Algorithm

• Treponemal test first (screening):
  • EIA or CIA
• Followed by nontreponemal test (confirmation):
  • RPR or VDRL 1

The sensitivity of these tests varies by stage of infection:

• Primary syphilis: RPR sensitivity 88.5%, VDRL 82-100%
• Secondary syphilis: RPR and VDRL sensitivity approaches 100%
• Latent untreated: RPR sensitivity 95.7%
• Late latent syphilis: RPR and VDRL sensitivity 61-75% 1

HIV Blood Tests

HIV testing is conducted using a multi-step approach:

Initial Screening

• 4th generation combination tests (detect both HIV-1/HIV-2 antibodies and HIV-1 p24 antigen)
  • Sensitivity: 100%
  • Specificity: 99.99%
  • Can detect infection 4-7 days earlier than antibody-only tests 2

Confirmatory Testing

• HIV-1/HIV-2 antibody differentiation immunoassay
• If negative, proceed to nucleic acid testing (NAT) for HIV-1 RNA
• Traditional Western blot or immunofluorescence assay (IFA) may still be used in some settings 2

Other Blood Tests for STDs

Hepatitis B

• Hepatitis B surface antigen (HBsAg) - recommended for all pregnant women at first prenatal visit
• Should be repeated late in pregnancy for high-risk women who initially test negative 1

Hepatitis C

• Screening recommended for high-risk populations, including MSM (men who have sex with men) 1

Testing Recommendations by Population

General Population

• Syphilis: Not universally recommended for heterosexual males or non-pregnant females 1
• HIV: Recommended for sexually active patients aged 13-64 seeking evaluation for STIs 1

Pregnant Women

• Syphilis: All pregnant women should be screened at first prenatal visit
  • High-risk women should be rescreened in the third trimester and at delivery
  • No infant should be discharged without the mother's syphilis status being determined 1
• HIV: Should be offered to all pregnant women at first prenatal visit 1
• Hepatitis B: All pregnant women at first prenatal visit 1

Men Who Have Sex With Men (MSM)

• Syphilis: At least annual screening recommended
• More frequent screening (every 3-6 months) for high-risk individuals 1

Important Clinical Considerations

• False positives: Biological false positive rates for nontreponemal tests are higher in certain populations:

  • HIV-positive individuals: 10.7% (vs 4.2% in HIV-negative)
  • HBV-positive individuals: 8.3% (vs 3.7% in HBV-negative) 1

• Window period: HIV antibody tests may not detect infection that occurred less than 6 months before testing 1

• Partner notification: Critical for preventing reinfection and further transmission 1

Common Pitfalls to Avoid

1. Relying solely on clinical diagnosis - neither sensitive nor specific for most STDs 1

2. Missing co-infections - 25-40% of genital infections may not be specifically identified; testing for multiple STDs is often necessary 1

3. Failing to retest after treatment - especially important for trichomoniasis in HIV-infected females 1

4. Misinterpreting discordant syphilis test results - particularly with the reverse sequence algorithm, where 56.7% of treponemal EIA/CIA reactive sera had nonreactive RPR tests 3

5. Neglecting partner testing/treatment - patients who "fail" therapy often have been reinfected by untreated partners 1