RPR Accuracy at Six Weeks
The RPR test is highly accurate at 6 weeks post-infection, with sensitivity of 97-100% for secondary syphilis and 88.5% for primary syphilis, making it an excellent diagnostic tool at this timepoint. 1
Test Performance at Six Weeks
At 6 weeks (42 days) after infection, the RPR test demonstrates excellent sensitivity because:
Treponemal antibodies typically appear 1-4 weeks after infection, while nontreponemal antibodies (detected by RPR) appear slightly later but are reliably positive by 4-6 weeks in primary syphilis. 1
By 6 weeks, most patients have progressed from primary to secondary syphilis or are in late primary stage, where RPR sensitivity reaches 97-100%. 1
Testing at 6 weeks (and certainly by 9 weeks) is more than adequate to detect syphilis if infection had occurred. 1
Stage-Specific Sensitivity
The accuracy of RPR varies significantly by disease stage:
- Primary syphilis: 62-78% to 88.5% sensitivity 1, 2
- Secondary syphilis: 97-100% sensitivity 1, 2
- Early latent syphilis: 85-100% sensitivity 1
- Late latent syphilis: 61-76% sensitivity 1, 3
Clinical Implications for Six-Week Testing
At 6 weeks post-exposure, most infections will be detected because patients are typically in late primary or early secondary stage, where RPR sensitivity is highest. 1
Important Caveats:
RPR specificity is approximately 87.3-90.6%, meaning false positives can occur in pregnancy, autoimmune diseases, HIV infection, hepatitis C, and intravenous drug use. 2
All positive RPR results require confirmation with treponemal-specific testing (FTA-ABS, TP-PA, or treponemal EIA/CLIA) to distinguish true infection from biological false positives. 1, 3
RPR titers ≥1:8 are highly specific for true syphilis infection, with studies showing no false positives at this threshold. 1
Diagnostic Algorithm at Six Weeks
When testing at 6 weeks post-exposure:
Order both RPR (quantitative) and a treponemal test for complete diagnosis, as a single test type is insufficient. 3
If RPR is negative but clinical suspicion remains high, repeat testing in 2-4 weeks, as 11.5-37.5% of primary syphilis cases may still be RPR-negative. 1, 4
Consider direct detection methods (darkfield microscopy, direct fluorescent antibody testing) if lesions are present and initial serology is negative. 1, 3
Common Pitfalls
Do not rely solely on RPR for diagnosis—both nontreponemal and treponemal tests must be reactive to confirm syphilis. 3
HIV-infected patients may have atypical serologic responses with unusually low, high, or fluctuating titers, though standard tests remain accurate for most. 1, 3
Always request quantitative titers (e.g., 1:4,1:16,1:64), not just "positive/negative," as titers are critical for monitoring disease activity. 1