Syphilis Testing: Understanding Blood Tests and Detection

Routine blood tests (CBC, CMP) do not detect syphilis. Specific serologic tests are required:

Two-Step Testing Algorithm

1. Nontreponemal Test (Screening): RPR (Rapid Plasma Reagin) or VDRL. Detects anti-cardiolipin antibodies. Quantitative (titer). Sensitivity ~78–86% primary, ~100% secondary. False positives: pregnancy, autoimmune, infections, aging.
2. Treponemal Test (Confirmation): FTA-ABS, TP-PA, EIA/CIA, chemiluminescence. Detects specific anti-T. pallidum antibodies. Qualitative (reactive/non-reactive). Remains positive for life even after cure.

Interpretation

• RPR+ / Treponemal+ → Active or past infection. Check titer trend.
• RPR– / Treponemal+ → Past treated infection, late latent, or false-positive treponemal.
• RPR+ / Treponemal– → Likely false-positive RPR; repeat treponemal.

Direct Detection

Dark-field microscopy (primary chancre), PCR (lesion swab, CSF) – for early/congenital/neurosyphilis.