What lab tests should be ordered to diagnose syphilis?

Diagnostic Laboratory Tests for Syphilis

To diagnose syphilis, both nontreponemal and treponemal antibody tests should be ordered, as a single test is not sufficient for diagnosis. 1

Traditional vs. Reverse Screening Algorithms

Traditional Algorithm

1. Initial screening: Nontreponemal test

   • Rapid Plasma Reagin (RPR) or Venereal Disease Research Laboratory (VDRL)
   • Sensitivity: 50-92.7%, Specificity: 95-100% 1

2. Confirmatory test (if nontreponemal test is positive):

   • Treponemal-specific test such as:
     • T. pallidum particle agglutination (TP-PA)
     • Fluorescent treponemal antibody absorption (FTA-ABS)
     • Enzyme immunoassay (EIA)
     • Chemiluminescent immunoassay
   • Sensitivity: 95-100%, Specificity: 95-100% 1

Reverse Algorithm (Increasingly Common)

1. Initial screening: Treponemal test

   • Automated treponemal EIA or chemiluminescent immunoassay

2. Confirmatory test (if treponemal test is positive):

   • Nontreponemal test (RPR or VDRL)

3. Discrepant resolution (if treponemal positive but nontreponemal negative):

   • Second treponemal test (different from the first)
   • TP-PA is often used for this purpose 2, 1

Interpretation of Test Results

Nontreponemal Result	Treponemal Result	Interpretation
Positive	Positive	Confirmed syphilis (current or past)
Positive	Negative	Likely false-positive nontreponemal test
Negative	Positive	Possible very early infection, previously treated syphilis, or late-stage syphilis
Negative	Negative	No evidence of syphilis [1]

Specific Tests to Order

Nontreponemal Tests

• RPR (Rapid Plasma Reagin): Most sensitive nontreponemal test 3
• VDRL (Venereal Disease Research Laboratory)

Treponemal Tests

• TP-PA (T. pallidum particle agglutination): High sensitivity and specificity 3
• FTA-ABS (Fluorescent treponemal antibody absorption)
• EIA (Enzyme immunoassay): TrepSure Anti-Treponema EIA Screen has high sensitivity and specificity 3
• Automated treponemal tests: Liaison Treponema Screen and Architect Syphilis TP (high sensitivity, with Liaison having higher specificity) 3

Important Considerations

1. Quantitative testing: Always request quantitative nontreponemal tests (with titers) rather than qualitative results to:

   • Establish baseline for monitoring treatment response
   • Detect treatment failure (failure to achieve fourfold decrease in titer within 6-12 months) 1

2. Stage-specific sensitivity:

   • Primary syphilis: Nontreponemal tests may be negative in early primary stage
   • Secondary syphilis: Highest sensitivity for both test types
   • Late latent syphilis: Nontreponemal tests may become negative while treponemal tests remain positive 1, 4

3. False positives:

   • Nontreponemal tests can have false positives due to other conditions (autoimmune diseases, pregnancy, viral infections)
   • Using both test types minimizes misdiagnosis 2

4. Prozone phenomenon:

   • In secondary syphilis with very high antibody levels, nontreponemal tests may give false-negative results
   • Request dilution of samples if clinical suspicion is high despite negative results 2

5. Direct detection methods:

   • For primary syphilis with visible lesions, direct detection methods can be considered:
   • Darkfield microscopy of lesion exudate
   • PCR testing of lesion material 5

Follow-up Testing

After treatment, follow-up should include quantitative nontreponemal tests at 3,6,9,12, and 24 months, with clinical evaluation at 6 and 12 months to monitor treatment response 1.

The most recent CDC guidelines (2024) emphasize the importance of both test types used in conjunction for accurate diagnosis, as neither type alone is sufficient for definitive diagnosis of untreated syphilis 5.