TPHA (Treponema Pallidum Hemagglutination Assay)

Function

The TPHA test is a specific treponemal test used to detect antibodies directed against Treponema pallidum, the causative agent of syphilis. Unlike non-treponemal tests (like RPR or VDRL), the TPHA measures antibodies that are specific to the bacteria itself.

Why it is Ordered

It is primarily used as a confirmatory test following a positive screening result from a non-treponemal test. It is also used to diagnose syphilis in later stages when non-treponemal tests might become negative. Because TPHA antibodies usually persist for life, it is an excellent marker for determining if a patient has ever been infected.

Associated Conditions

• Primary Syphilis: TPHA may become positive within 1–4 weeks after the appearance of a chancre.
• Secondary Syphilis: The test is almost 100% sensitive during this stage of active systemic infection.
• Latent and Tertiary Syphilis: TPHA remains highly sensitive and is often the primary way to identify historical or late-stage infection.
• Congenital Syphilis: Used to evaluate infants born to mothers with a history of infection.

Summary

TPHA is a cornerstone of syphilis diagnostics. While it cannot distinguish between an active infection and a successfully treated past infection, its high specificity makes it invaluable for confirming the presence of the disease and preventing false positives associated with other conditions.