TPHA / FTA-ABS

Both the Treponema pallidum haemagglutination assay and the fluorescent Treponema antibody tests are highly specific for Treponema antigens:

• TPHA - sheep red blood cells coated with T. pallidum are agglutinated by patient's antibody
• FTA-ABS - T. pallidum is fixed to a microscope slide, antibodies in patient's serum attach and are detected by the addition of fluorescent anti-human immunoglobulin. The test is positive in 90% of patients with primary infection and positive in all patients with secondary or tertiary infection. It can be adapted to detect either IgG or IgM antibody. This test becomes positive in early disease (at about 3-4 weeks after infection - at the same time as the reagin tests).

False positives may result from non-sexually transmitted treponemal diseases of the tropics - such as Yaws and Pinta. Differentiation requires a careful history and examination.

In some texts the THPA is referred to as the microhaemagglutination assay for antibodies to Treponema pallidum.