FLUORESCENT TREPONEMAL ANTIBODY ABSORPTION

The fluorescent treponemal antibody absorption (FTA-ABS) test measures a specific antibody made against Treponema pallidum, the bacterium that causes syphilis. The test is reserved for confirmation of a positive screening test for syphilis and distinguishes patients with true infection from those with a false positive result of a screening test. Once a person tests positive, he or she will usually test positive for life. Therefore, the test cannot be used to measure disease activity or differentiate past from present infection.

The FTA-ABS is performed by first heating a patient's serum and mixing it in an extract of nonpathogenic treponemes called "sorbent." This step removes any cross-reacting antibodies that may have developed against treponemes that naturally reside in the human mouth or genital tract. The serum is then layered onto slides containing T. pallidum. Anti-human antibodies labeled with a fluorescent indicator are added, and the slides are examined under a fluorescent microscope. The intensity of fluorescence is quantified using a one (weakly positive) to four (strongly positive) scale. Though very sensitive and highly specific for syphilis, this test tends to be expensive, subjective, and time-consuming, as it requires interpretation by an experienced technician.

Judith E. Wolf

(see also: Antibody, Antigen; RPR Test; Syphilis; VDRL Test )

Bibliography

Tramont, E. (2000). "Treponema Pallidum." In Principles and Practice of Infectious Diseases, eds. G. Mandell, J. Bennett, and R. Dolin. Philadelphia, PA: Churchill Livingstone.