The fluorescent treponemal antibody absorption (FTA-ABS) test is a diagnostic test for syphilis. Using antibodies specific for the Treponema pallidum species, such tests are more specific than non-treponemal testing such as VDRL. In addition, FTA-ABS turns positive earlier and remains positive longer than VDRL. Other treponemes, such as T. pertenue, may also produce a positive FTA-ABS.
FTA-ABS should always be followed to confirm a positive RPR and/or VDRL test for syphilis. The ABS suffix refers particularly to a processing step used to remove nonspecific antispirochetal antibodies present in normal serum.
The antigen for the FTA-ABS test is whole bacteria. The bacteria cannot be cultured on laboratory media, so the organisms used are a lyophilized suspension of T. pallidum extracted from rabbit testicular tissue. This is spread over and fixed to a slide. Patient serum is mixed with an absorbent (the "ABS" part of the test) containing an extract of a non-pathogenic treponeme, Treponema phagedenis biotype Rieter. The purpose of the absorbent is to remove anti-treponemal antibodies that are not specific for the syphilis bacteria. The pre-adsorbed patient serum is then added to the slide; if the patient has been infected by syphilis, their antibodies will bind to the bacteria. FITC (a fluorophore)-labeled anti-treponeme antibody and TRITC (another fluorophore)-labeled anti-human antibodies are added as secondary antibodies. The spirochete location is identified using the FITC staining and the TRITC staining identifies whether the patient has anti-T. pallidum antibodies (binding to the same spirochete).