FTA-Abs is a treponemal test for syphilis

Syphilis

Syphilis is a sexually transmitted infection caused by the spirochete bacterium Treponema pallidum subspecies pallidum. The primary route of transmission is through sexual contact; however, it may also be transmitted from mother to fetus during pregnancy or at birth, resulting in congenital syphilis...

. Using antibodies specific for the Treponema pallidum

Treponema pallidum

Treponema pallidum is a species of spirochaete bacterium with subspecies that cause treponemal diseases such as syphilis, bejel, pinta and yaws. The treponemes have a cytoplasmic and outer membrane...

 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 (Fluorescent Treponemal Antibody) - Abs (absorption) should always be followed to confirm a positive RPR

Rapid Plasma Reagin

Rapid Plasma Reagin refers to a type of test that looks for non-specific antibodies in the blood of the patient that may indicate that the organism that causes syphilis is present...

 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.

Procedure

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

Fluorescein isothiocyanate

Fluorescein isothiocyanate is a derivative of fluorescein used in wide-ranging applications including flow cytometry. FITC is the original fluorescein molecule functionalized with an isothiocyanate reactive group , replacing a hydrogen atom on the bottom ring of the structure...

(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).

Utility

This test is not useful for following therapy, because it does not wane with successful treatment of the disease, and will continue to be positive for the life of the patient after primary exposure.