For each stage of syphilis (primary, secondary, tertiary), what are the expected rapid plasma reagin (RPR) screening results and titers, and the Treponema pallidum particle agglutination (TP‑PA) results?

Syphilis Serologic Test Results by Stage

For primary syphilis, RPR is reactive in approximately 78–86% of cases with variable titers, and TP-PA is reactive in 82–91%; for secondary syphilis, both RPR (reactive in 95–100% with typically high titers) and TP-PA (reactive in nearly 100%) are positive; for tertiary syphilis, RPR may be reactive in only 70–73% of cases with low titers, while TP-PA remains reactive in the vast majority of patients. 1, 2, 3

Primary Syphilis

• RPR Screen: Reactive in 78–86% of cases 1, 3

  • RPR Titer: Variable, often 1:2 to 1:32; may be negative in very early infection (first 1–3 weeks after chancre appearance) 1, 2
  • A seronegative window period exists in early primary syphilis when direct detection methods (dark-field microscopy) are required for diagnosis 2, 4

• TP-PA: Reactive in 82–91% of cases 1, 3

  • Treponemal tests typically turn reactive slightly earlier than nontreponemal tests 5
  • The 47-kDa T. pallidum antibody is detectable in all primary syphilis cases by Western blot, even when RPR and TP-PA may be negative 6

Secondary Syphilis

• RPR Screen: Reactive in 95–100% of cases 1, 4, 7

  • RPR Titer: Typically high, often ≥1:16 to ≥1:32 1, 2, 8
  • Prozone phenomenon can rarely produce false-negative results when antibody titers are extremely high; dilution resolves this 2, 5

• TP-PA: Reactive in nearly 100% of cases 1, 3

  • Sensitivity approaches 100% in secondary syphilis 5, 3

Tertiary Syphilis

• RPR Screen: Reactive in approximately 70–73% of cases 1, 4

  • RPR Titer: Typically low (often 1:1 to 1:8) or may be nonreactive 1, 2
  • Nontreponemal antibody titers decline over time in untreated late syphilis, leading to lower sensitivity 1, 3

• TP-PA: Reactive in the vast majority of cases 1, 3

  • Treponemal tests remain positive for life in most patients, even in late-stage disease 2, 9
  • Only 15–25% of patients treated in the primary stage may show treponemal test reversion after 2–3 years 2, 9

Critical Testing Considerations

• Combined testing is mandatory: No single assay is sufficient; both nontreponemal (RPR/VDRL) and treponemal (TP-PA/FTA-ABS) tests must be used together for accurate diagnosis 2, 3

• Quantitative titers are essential: RPR titers must be reported with complete endpoint dilutions (not truncated at >1:32) to enable proper disease monitoring 1, 2

• Test consistency matters: Use the same nontreponemal assay and laboratory for serial monitoring, as RPR titers are generally slightly higher than VDRL and cannot be directly compared 2, 10

• HIV coinfection: Patients with HIV may show atypical serologic patterns (unusually high, low, or fluctuating titers), though serology remains reliable for the majority 2, 7

• Early latent syphilis (acquired within the past year): RPR reactive in >95% with titers similar to secondary syphilis; TP-PA reactive in nearly 100% 1, 2

• Late latent syphilis (>1 year duration or unknown): RPR reactive in 70–95% with lower titers; TP-PA remains reactive in the vast majority 1, 2, 3