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FTA-ABS Test: Principle, Procedure, Results and Interpretation

Positive FTA-Abs test result

Fluorescent treponemal antibody-absorption (FTA-ABS) test is an indirect fluorescent antibody technique used as a confirmatory test for syphilis. FTA-ABS is the first serological test (cardiolipin or specific treponemal) to become positive following infection i.e., 3-4 weeks after Infection.

This test is inexpensive confirmatory test performed in reference public health laboratories. FTA-ABS test results is positive in most patients with primary syphilis and the test result remains positive for life so FTA-ABS test for syphilis cannot be used to determine the response to treatment.

FTA-ABS Test Principle 

Serum from patients suspected of having syphilis because of previous positive VDRL or RPR test  is diluted in 1:5 sorbent (patients’ serum is absorbed with treponemes other than Treponema pallidum to remove nonspecific antibodies).

Note: Sorbent is an extract from cultures of Treponema phagedenis, Reiter treponeme.

This absorbed serum is layered on a microscope slide which contains  fixed T. pallidum subspecies pallidum. If the patient’s serum contains antibody, the antibody will coat the treponeme. Next, fluorescein isothiocyanate (FITC)-labeled antihuman immunoglobulin is added; this combines with the patient’s IgG and IgM antibodies that are adhering to T. pallidum, and results in a visible test reaction when examined by fluorescence microscopy. FTA-ABS test should not be used as a primary screening procedure.

Sensitivity of Fluorescent Treponemal Antibody Test

  1. Primary Syphilis: 85%
  2. Secondary Syphilis: 100%
  3. Late Syphilis: 98%

Specimen: Serum is the most appropriate specimen for the FTA-ABS test; however, spinal fluid may be used.

Sample Acceptance/Rejection Criteria: 

Note: When an unsatisfactory sample is received in the laboratory, notify the requesting physician and discuss whether testing is appropriate for that specimen.

Materials/Reagents :
(These reagents can be purchased from commercial suppliers in ready to use form; read insert or description/manual of the suppliers for detail information; other routine instruments required in lab has not been listed here)

For detail information about Procedure and Reporting please download and read this document of CDCFluorescent Treponemal Antibody-Absorption (FTA-ABS test)