FTA-ABS Test: Principle, Procedure, Results

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 to become positive following infection i.e., 3-4 weeks after Infection.

This test is inexpensive confirmatory test performed in reference to public health laboratories. FTA-ABS test results are positive in most patients with primary syphilis and the test result remains positive for life so the 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 a specific 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%

Sample Acceptance/Rejection Criteria: 

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

• Specimen should not contain particulate matter that would interfere with reading test results.
• Specimens that are excessively hemolyzed (i.e. when printed matter cannot be read through it), grossly contaminated with bacteria, chylous, or otherwise extremely turbid are unsatisfactory.
  Note. Hemolysis may be caused by transporting blood in freezing or extremely hot weather without proper insulation.
• An acceptable spinal fluid specimen must be crystal clear. Any visible tinge of blood may lead to invalid results

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 the insert or description/manual of the suppliers for detailed information; other routine instruments required in a lab has not been listed here.

• Treponema pallidum antigen (Suspension of T. pallidum-Nichols strain)
• FITC-labeled antihuman immunoglobulin
• Sorbent (cultures of nonpathogenic Reiter treponemes)
• Reactive control serum (a pool of human serum is obtained from syphilitic donors that are 4+ reactive)
• Nonspecific control serum (the nonspecific control serum is a serum pool obtained from individuals without syphilis).
• Immersion oil
• Acetone

For detailed information about Procedure and Reporting please download and read this document of CDC: Fluorescent Treponemal Antibody-Absorption (FTA-ABS test)