What test is done after a positive Rapid Plasma Reagin (RPR) test?

Confirmatory Testing After a Positive RPR

After a positive Rapid Plasma Reagin (RPR) test, a treponemal test such as TP-PA (Treponema pallidum particle agglutination), FTA-ABS (fluorescent treponemal antibody-absorbed), or another treponemal immunoassay should be performed to confirm the diagnosis of syphilis. 1

Diagnostic Algorithm for Syphilis

1. Initial screening with RPR (non-treponemal test)

   • If positive → Proceed to confirmatory treponemal test
   • RPR sensitivity: 50-92.7%, specificity: 95-100% 1
   • Note: RPR has limited sensitivity (62-78%) in primary syphilis 1

2. Confirmatory testing options (treponemal tests):

   • TP-PA (T. pallidum particle agglutination) - Sensitivity: 95-100%, Specificity: 95-100% 1
   • FTA-ABS (Fluorescent treponemal antibody-absorption) - Sensitivity: 95-100%, Specificity: 95-100% 1
   • Treponemal immunoassays (EIA or chemiluminescent assays) 2

Important Considerations

• Treponemal tests detect antibodies specific to T. pallidum and remain positive for life in most patients, even after successful treatment 1
• Non-treponemal tests like RPR measure antibodies to cardiolipin and can be used to monitor treatment response 1
• False-positive RPR results are more common in:
  • Women (0.27% vs 0.20% in men)
  • Patients over 60 years (0.34% vs 0.25% in younger patients) 1

Alternative Testing Approach

Some laboratories use a reverse sequence algorithm:

1. Start with a treponemal test
2. If positive, follow with a non-treponemal test (RPR)
3. If discordant results (treponemal reactive/non-treponemal nonreactive), perform a second treponemal test to rule out false positives 1

Special Situations

• Neurosyphilis evaluation: If late latent syphilis, CD4 count <350 cells/mm³ with high-titer VDRL (>1:32), or treatment failure is present, consider lumbar puncture with CSF-VDRL testing 1
• CSF testing: For suspected neurosyphilis, both VDRL and treponemal tests (TP-PA or FTA-ABS) may be performed on CSF 2, 3
  • CSF-VDRL has high specificity but limited sensitivity for neurosyphilis 1

Follow-up Testing

After treatment, quantitative non-treponemal tests (RPR or VDRL) should be used to monitor response:

• Expected fourfold decline in titers within 6 months for primary/secondary syphilis
• Follow-up testing recommended at 3,6,9, and 12 months after treatment 1

Pitfalls to Avoid

• Do not rely solely on non-treponemal tests for diagnosis due to their limited sensitivity, especially in primary syphilis
• Treponemal tests can remain positive for life, so they cannot distinguish between current and past treated infections
• FTA-ABS, while highly sensitive, can occasionally give equivocal or false-negative results 4
• Failure to confirm a positive RPR with a treponemal test may lead to misdiagnosis and inappropriate treatment

Remember that proper diagnosis of syphilis requires correlation of laboratory findings with clinical presentation and history of exposure.