Fluorescent treponemal antibody absorption test

Also called FTA-ABS test, The fluorescent treponemal antibody absorption is a diagnostic test for syphilis.

How is the test performed?

It is a blood test.

Science

Using antibodies specific for the Treponema pallidum species, active syphilis infection can be detected.

Types

There are two types of FT ABS tests, VDRL and RPR

• A presumptive diagnosis of syphilis requires use of two tests: a nontreponemal test (i.e., Venereal Disease Research Laboratory test [VDRL] or Rapid Plasma Reagin test [RPR]) and a treponemal test (i.e., fluorescent treponemal antibody absorbed [FTA-ABS] tests, the T. pallidum passive particle agglutination [TP-PA] assay, various enzyme immunoassays [EIAs], chemiluminescence immunoassays, immunoblots, or rapid treponemal assays).
• Use of only one type of serologic test is insufficient for diagnosis and can result in false-negative results in persons tested during primary syphilis and false-positive results in persons without syphilis.

Monitor treatment response

FTS-ABS test titers might correlate with disease activity and are used to follow treatment response. Results should be reported quantitatively. A fourfold change in titer, equivalent to a change of two dilutions (e.g., from 1:16 to 1:4 or from 1:8 to 1:32), is considered necessary to demonstrate a clinically significant difference between two nontreponemal test results obtained using the same serologic test. Sequential serologic tests in individual patients should be performed using the same testing method (VDRL or RPR), preferably by the same laboratory.

VDRL versus RPR

The VDRL and RPR are equally valid assays, but quantitative results from the two tests cannot be compared directly because RPR titers frequently are slightly higher than VDRL titers.

VDRL slide used in VDRL test

Positive FT ABS test

A positive screening test for syphilis using either VDRL or RPR methods means you have a current syphilis infection.

Normal FT ABS test

A negative or nonreactive result means that you do not have a current or past infection with syphilis.

False positives

• Other illnesses, such as yaws and pinta may also result in positive FTA-ABS results.
• False-positive nontreponemal test results can be associated with various medical conditions and factors unrelated to syphilis, including other infections (e.g., HIV), autoimmune conditions, immunizations, pregnancy, injection-drug use, and older age (395,396).
• Persons with a reactive nontreponemal test should always receive a treponemal test to confirm the diagnosis of syphilis.

Other names

FTA ABS test