What this test measures
Detects IgG / IgM antibodies against Treponema pallidum-specific antigens (TpN15, TpN17, TpN47), usually by ELISA or chemiluminescence. Highly specific for syphilis. Once positive, almost always remains positive for life — cannot be used to monitor treatment response (use VDRL/RPR for that). In modern WHO-recommended "reverse algorithm", TPAb is the FIRST-line screening test; reactive results trigger confirmatory VDRL/RPR.
Why it matters
In Indian practice, TPAb is increasingly the first-line antenatal screen at well-resourced centres, replacing the traditional VDRL → TPHA workflow. Advantages: higher sensitivity in early and late syphilis, automated high-throughput assays, fewer biological false positives. Disadvantages: cannot distinguish active from past treated infection, doesn't track treatment success.
How to prepare
No fasting. Random sample. Disclose any past syphilis treatment (the test remains positive for life), pregnancy, HIV status, IV drug use, and prior antibiotic use.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Treponemal Antibody (reactive / non-reactive (or S/CO ratio))[1] | Non-reactive (S/CO < 1.0) | Non-reactive — no syphilis infection. Very early primary syphilis can be missed (window 2–6 weeks). | Reactive — confirm with VDRL/RPR (quantitative titre). Reactive TPAb + reactive VDRL = active or past treated syphilis (stage by history + titre). Reactive TPAb + non-reactive VDRL = old treated infection, very early, or late latent. |
Reverse syphilis algorithm interpretation
| TPAb | VDRL/RPR | Interpretation |
|---|---|---|
| Non-reactive | — | No syphilis (re-test if window period) |
| Reactive | Reactive | Active or past treated — clinical staging needed |
| Reactive | Non-reactive | Past treated, late latent, or very early |
| Reactive | Reactive (high titre, rising) | Active untreated or reinfection |
Frequently asked questions
Why does TPAb stay positive forever?
Treponemal antibodies (IgG class) persist for life after exposure, even after curative treatment. This is why VDRL/RPR (which DOES fall with treatment) is used to monitor response, not TPAb.
What is the difference between TPAb and VDRL?
TPAb is the specific (treponemal) test — detects antibodies against T. pallidum proteins. VDRL is the non-specific (non-treponemal) test — detects antibodies against cardiolipin. Both have their roles; modern algorithms use both.
Is there any false-positive TPAb?
Very rare — TPAb is highly specific. Cross-reactivity with non-syphilitic treponemes (yaws, pinta, bejel — rare in India) is possible. Some labs report borderline / equivocal results that need a second specific test (TPHA, FTA-ABS).
Should pregnant women have this test?
Yes — universal antenatal syphilis screening is recommended. Reactive results need confirmation and prompt treatment to prevent congenital syphilis.
I had syphilis 20 years ago and was treated — will TPAb still be positive?
Yes, almost certainly. The TPAb result alone cannot say "active" — VDRL/RPR will be non-reactive or very low if treatment was successful.
How accurate is the test?
TPAb has 95–99% sensitivity and 99% specificity in active and latent syphilis. Very early primary infection (< 2 weeks) is the main miss.
Related HIV / STI tests
Tests commonly ordered alongside TREPONEMA PALLIDUM ANTIBODY (TPAB), or that help interpret an unexpected result.
Sources & references
- CDC — Syphilis Laboratory Diagnosis · accessed 2026-05-30T00:00:00.000Z
- WHO — Syphilis Strategy · accessed 2026-05-30T00:00:00.000Z
- NACO India — Syphilis Diagnosis · accessed 2026-05-30T00:00:00.000Z
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