What is the next step in workup for a patient with a positive Fluorescent Treponema Antibody (FTA) screen?

Next Steps in Workup for Positive Fluorescent Treponema Antibody (FTA) Screen

For a patient with a positive Fluorescent Treponema Antibody (FTA) screen, the next step in workup should be to perform a quantitative non-treponemal test such as RPR (Rapid Plasma Reagin) or VDRL (Venereal Disease Research Laboratory) to assess disease activity and guide treatment decisions. 1

Diagnostic Algorithm for Syphilis

1. Initial Positive Treponemal Test (FTA-ABS)

   • FTA-ABS is a highly sensitive (95-100%) treponemal test that detects antibodies against T. pallidum 1
   • A positive result indicates current or past infection but cannot distinguish between active and treated disease

2. Follow-up Non-Treponemal Testing

   • Perform quantitative RPR or VDRL test immediately
   • These tests detect antibodies generated during active infection and can be used to monitor treatment response
   • Sensitivity of non-treponemal tests ranges from 50-92.7% with specificity of 95-100% 1

3. Interpretation of Combined Results

   • Positive treponemal + Positive non-treponemal: Indicates active or recently treated syphilis
   • Positive treponemal + Negative non-treponemal: May indicate:
     • Previously treated syphilis
     • Very early infection
     • Late/latent syphilis with low titers
     • False positive treponemal test

Additional Workup Considerations

• Clinical evaluation for signs/symptoms of syphilis (primary chancre, secondary rash involving palms/soles, lymphadenopathy, constitutional symptoms) 2, 1

• HIV testing due to high co-infection rates and potential impact on syphilis presentation and progression 1

• If neurosyphilis is suspected (based on neurological symptoms, HIV status, or high-risk features):

  • CSF analysis for VDRL, cell count, and protein 2
  • Neurologic examination

• For pregnant patients with positive serology:

  • Immediate treatment is critical regardless of stage
  • Additional fetal evaluation may be needed 2

Common Pitfalls and Caveats

1. Reverse screening algorithm confusion: Many laboratories now use treponemal tests like FTA-ABS as initial screening followed by non-treponemal tests, which is the reverse of traditional algorithms 3. Understanding this sequence is critical for proper interpretation.

2. False positives: Treponemal tests can remain positive for life even after successful treatment. Non-treponemal tests help distinguish active from past infection 1.

3. Prozone phenomenon: In some cases of secondary syphilis, high antibody levels can cause false-negative non-treponemal tests. If clinical suspicion is high despite negative RPR/VDRL, request dilution of the specimen.

4. Serofast state: Some patients maintain low-positive non-treponemal titers despite adequate treatment. This is not treatment failure but requires clinical correlation.

5. Early infection window: Very early syphilis may have negative serology. If clinical suspicion is high, consider direct detection methods like darkfield microscopy or direct fluorescent antibody tests of lesion exudate 2, 1, 4.

The CDC diagnostic algorithm recommends that after a positive treponemal test like FTA-ABS, quantitative non-treponemal testing is essential to guide clinical decision-making and establish a baseline for monitoring treatment response 1.