Which serological test becomes positive earliest in primary syphilis and detects antibodies against treponemal antigens?

Explanation

## Serological Diagnosis of Syphilis: Timing and Test Categories **Key Point:** Syphilis serology is divided into two categories: **non-treponemal** (RPR, VDRL, TRUST) and **treponemal** (FTA-ABS, TP-PA, EIA). The **FTA-ABS test** detects specific antibodies against T. pallidum antigens and becomes positive earliest in primary syphilis. ### Timeline of Serological Positivity | Test Type | Test Name | Becomes Positive | Remains Positive | Detects | |-----------|-----------|------------------|------------------|----------| | **Treponemal** | FTA-ABS | **3–4 weeks** (earliest) | Lifelong | Anti-treponemal IgG/IgM | | **Treponemal** | TP-PA | 3–5 weeks | Lifelong | Anti-treponemal antibodies | | **Non-treponemal** | RPR | 4–6 weeks | Variable (may become negative with treatment) | Cardiolipin (reagin) | | **Non-treponemal** | VDRL | 4–6 weeks | Variable | Cardiolipin (reagin) | | **Non-treponemal** | TRUST | 4–6 weeks | Variable | Cardiolipin (reagin) | **High-Yield:** In **primary syphilis**, the FTA-ABS becomes positive 3–4 weeks after infection (during the chancre stage), while non-treponemal tests (RPR/VDRL) may still be negative or weakly positive. This makes FTA-ABS the most sensitive test for early primary syphilis. **Mnemonic:** **"Non-treponemal tests are Non-specific (NTN)"** — they detect cardiolipin (a host antigen), not treponemal antigens. **"Treponemal tests are Treponemal-specific (TTS)"** — they detect antibodies against T. pallidum. ### Diagnostic Algorithm ```mermaid flowchart TD A[Suspected Syphilis]:::outcome --> B[Screen with RPR/VDRL<br/>Non-treponemal]:::action B --> C{RPR/VDRL<br/>Positive?}:::decision C -->|Yes| D[Confirm with FTA-ABS<br/>or TP-PA<br/>Treponemal]:::action C -->|No| E[Syphilis unlikely<br/>Consider other diagnoses]:::outcome D --> F{Treponemal<br/>Positive?}:::decision F -->|Yes| G[Syphilis confirmed<br/>Determine stage]:::outcome F -->|No| H[False-positive RPR<br/>Autoimmune/other cause]:::outcome ``` **Clinical Pearl:** A positive FTA-ABS in the absence of clinical signs does not always mean active disease — it may indicate past infection or latent syphilis. Always correlate serology with clinical findings and non-treponemal titers.