TPHA

TPHA, or Treponema pallidum Hemagglutination Assay, is a serologic test used to detect antibodies to Treponema pallidum, the bacterium that causes syphilis. It is a treponemal test typically used as a confirmatory assay after a positive non-treponemal screening test such as the rapid plasma reagin (RPR) or Venereal Disease Research Laboratory (VDRL) test.

Principle and method: The test uses red blood cells coated with Treponema pallidum antigens. If anti-treponemal antibodies are present in the patient serum, the coated cells agglutinate, producing a visible positive result. If no antibodies are present, no agglutination occurs, yielding a negative result. The assay can be performed in microtiter plates or in tube formats with appropriate positive and negative controls.

Interpretation and clinical significance: A positive TPHA indicates exposure to Treponema pallidum, either current or past infection. Treponemal tests, including TPHA, often remain reactive for life even after successful treatment, so TPHA is not used to measure treatment response. A negative result does not exclude very early infection, but in the appropriate clinical context it supports ruling out syphilis when combined with non-treponemal test results.

Clinical use and limitations: TPHA is widely used to confirm syphilis following a positive non-treponemal screen and as part of some testing algorithms. It is highly specific but does not distinguish active from previously treated infection. False positives are rare but can occur, and TPHA does not provide quantitative data for monitoring disease activity.