# Syphilis Serological Diagnosis

> Syphilis serological diagnosis for NEET-PG Microbiology, covering VDRL, RPR, TPHA, FTA-ABS, biological false positives, prozone effect and the clinical stages.

- Canonical URL: https://prepelephant.com/topics/neet-pg/microbiology/syphilis-serological-diagnosis
- Exam / course: NEET-PG · Subject: Microbiology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-14
- Last updated: 2026-10-14
- Reference basis: Ananthanarayan and Paniker's Textbook of Microbiology; Harrison's Principles of Internal Medicine; Medical Microbiology
- How to cite: "Syphilis Serological Diagnosis", PrepElephant, https://prepelephant.com/topics/neet-pg/microbiology/syphilis-serological-diagnosis

## Direct answer

Syphilis serology uses two families of tests to detect infection with Treponema pallidum: non-treponemal tests such as VDRL and RPR, and treponemal tests such as TPHA and FTA-ABS. Non-treponemal tests detect antibodies to cardiolipin and are used for screening and for monitoring treatment, while treponemal tests detect antibodies to the organism itself and usually stay positive for life. A reactive non-treponemal result must therefore be confirmed by a treponemal test.

## What you must remember

- **VDRL:** a non-treponemal flocculation test using a cardiolipin-lecithin-cholesterol antigen, and the preferred test on cerebrospinal fluid in neurosyphilis.
- **RPR:** a non-treponemal card test with charcoal particles, used for screening and for quantitative titres.
- **TPHA:** a treponemal haemagglutination test that is specific for syphilis and usually remains positive after cure.
- **FTA-ABS:** a treponemal fluorescent test that uses prior absorption with a non-pathogenic treponeme to remove cross-reacting antibodies.
- **Biological false positives:** acute causes include malaria and infectious mononucleosis, while chronic causes include leprosy, systemic lupus erythematosus and antiphospholipid syndrome.
- **Prozone phenomenon:** very high antibody levels in secondary syphilis can give a falsely negative non-treponemal result unless the serum is diluted.
- **Stages:** primary (chancre), secondary (rash with palmar and plantar involvement), latent (early under one year, late over one year), and tertiary (gummas, cardiovascular and neurosyphilis).
- **Treatment:** benzathine penicillin G 2.4 million units intramuscularly for early syphilis, and three weekly doses for late latent disease.

## Interpreting a reactive syphilis screen

A reactive non-treponemal test is common and does not by itself confirm syphilis. The steps below separate true infection from false positives and then guide the monitoring plan.

1. **Screen:** a VDRL or RPR test is performed, and a reactive result gives a titre that is recorded for later comparison.
2. **Confirm:** a treponemal test such as TPHA, TP-PA or an enzyme immunoassay is positive in true syphilis, which settles the diagnosis.
3. **Check for prozone effect:** a surprisingly high titre or a negative screen with clinical signs prompts dilution and retesting of the serum.
4. **Exclude a biological false positive:** a reactive non-treponemal test with a negative treponemal test points to a false positive, so the clinician looks for infection, pregnancy, autoimmune disease or drug causes.
5. **Stage the disease:** history, examination and an estimate of the duration of infection decide the regimen and the need for neurological assessment.
6. **Monitor:** a fourfold fall in the non-treponemal titre over several months signals response, while the treponemal result is not used to judge cure.

## Where the exam tries to catch you

Syphilis serology questions turn on which test to trust, which to follow and what a false result looks like.

- **Trap:** believing a reactive VDRL confirms syphilis — **Correct:** a non-treponemal reaction must be confirmed with a treponemal test, because biological false positives are common.
- **Trap:** using TPHA titres to judge response to treatment — **Correct:** treponemal tests stay positive after cure, so response is followed with the falling non-treponemal titre.
- **Trap:** accepting a negative VDRL in secondary syphilis without thinking of prozone — **Correct:** very high titres can read falsely negative, so dilution testing resolves the discrepancy.

## Frequently asked questions

### Why is FTA-ABS used to confirm syphilis?
FTA-ABS is a treponemal test that is highly specific for syphilis and is used to confirm a reactive non-treponemal screen. Its prior absorption step removes cross-reacting antibodies, so a reactive FTA-ABS result usually reflects true Treponema pallidum infection.

### What is a biological false positive in syphilis serology?
A biological false positive is a reactive non-treponemal test, such as VDRL or RPR, in a person who does not have syphilis. It occurs in infections such as malaria and leprosy and in autoimmune disease such as systemic lupus erythematosus, and a negative treponemal test distinguishes it from true syphilis.

### Which test is preferred for neurosyphilis in cerebrospinal fluid?
The VDRL test is the preferred non-treponemal test on cerebrospinal fluid in neurosyphilis, although its sensitivity is low. A negative CSF VDRL therefore does not exclude neurosyphilis, and the diagnosis combines CSF findings with the clinical picture.

### Why do treponemal tests remain positive after treatment for syphilis?
Treponemal tests such as TPHA and FTA-ABS detect antibodies to Treponema pallidum that persist long after the organism has been cleared. Response to treatment in syphilis is therefore judged by the fall in the non-treponemal titre rather than by the treponemal result.

### What is the prozone phenomenon in syphilis serology?
The prozone phenomenon is a false-negative non-treponemal result that occurs when antibody is present in very high concentration, as in some cases of secondary syphilis. Diluting the serum and retesting reveals the true titre and resolves the discrepancy.