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What to Know About the FTA-ABS Blood Test

Medically Reviewed by Dan Brennan, MD on May 27, 2021

The fluorescent treponemal antibody test absorption test (FTA-ABS) checks your blood for antibodies to the bacteria that causes syphilis called Treponema pallidum. Syphilis is a sexually transmitted disease (STD) that is spread by skin or mucous membrane contact with the sores of an infected person. This test is usually done after other screening tests for syphilis.

Why Is the FTA-ABS Blood Test Done?

The FTA-ABS blood test is usually done to confirm a positive syphilis result from another type of test. There are several different tests that doctors use to detect syphilis. 

The most common types of tests check your blood for antibodies. Your body develops antibodies to fight antigens. Antigens are substances like bacteria that trigger your immune system

Types of Syphilis Tests

The two main types of antibody tests that check for syphilis are nontreponemal antibody tests and treponemal antibody tests. A positive result on either type of test means you will need to have the other type as a follow-up to confirm. Nontreponemal antibody tests can tell you if you have an active infection.

You will have treponemal antibodies for the rest of your life once you've had syphilis. So the treponemal antibody tests can't tell whether you have an active infection or have had an infection in the past.

Nontreponemal antibody tests. These tests detect antibodies that are not specific to the Treponema pallidum bacteria. They include the Rapid Plasma Reagin (RPR) and the Venereal Disease Research Laboratory (VDRL) tests. They are sensitive but can lead to false positives since other conditions besides syphilis can cause your body to produce the antibodies they detect. 

Other illnesses and conditions that can produce a false positive include:

Treponemal antibody tests. These tests detect antibodies that are only produced in response to Treponema pallidum bacteria. They tend not to produce false-positive results. 

The FTA-ABS blood test is one of these. It can detect whether you've had syphilis but it can't detect antibodies until 3 to 4 weeks after you've been exposed. It's also used to test cerebrospinal fluid to diagnose neurosyphilis.  

Direct bacteria tests. Some tests detect the actual presence of Treponema pallidum bacteria in your system. They are also called darkfield microscopy and molecular testing. These are not used as often as other tests.

What Do Your Results Mean?

Normal. A normal or nonreactive result means that you don't currently have a syphilis infection and you've never been infected with the STD. 

Abnormal. An abnormal or positive result usually means you have an active syphilis infection or have had one before. Your FTA-ABS test result will be positive for the rest of your life, even after you've been treated. There are two other skin conditions called yaws and pinta that may cause a positive FTA-ABS test result. Sometimes women with lupus can have a false-positive result. 

How Is Syphilis Treated?

Syphilis can easily be treated in the early stages with antibiotics from your doctor. But it can cause serious or life-threatening health problems for you and your baby if you're pregnant. 

Treatment during the later stages of syphilis may not undo any damage that has already occurred.

Stages of Syphilis

Syphilis progresses through stages if untreated. The stages may overlap. You can have syphilis for years without having any symptoms. It's critical to get tested as soon as possible if you think you may have been exposed.

Primary stage. You will develop a chancre about 3 weeks after you are first exposed. You may not notice it because it can be in your vagina or rectum. You will usually only develop one chancre but you can develop more. They are usually painless and heal in 3 to 6 weeks.

Secondary stage.  A few weeks after the chancre heals, you may develop:

The symptoms of secondary syphilis may disappear entirely after a few weeks or they may return intermittently for up to a year.  

Latent stage. This is the hidden stage when there are no symptoms. This stage can last for years. Your symptoms might never return or you may progress to the tertiary stage.  

Tertiary stage. This is the late stage of syphilis. It develops in about 15% to 30% of people who aren't treated. It can occur many years after you were first exposed. In this stage syphilis can cause damage to your:

  • Eyes
  • Nerves
  • Brain
  • Liver
  • Bones
  • Joints
  • Heart
  • Blood vessels

Neurosyphilis. This can occur at any stage. Syphilis can spread and cause damage to your brain, nervous system, or eyes.    

Congenital syphilis. This occurs when you pass syphilis on to your baby. They can become infected in the placenta or during birth. Babies with syphilis can be premature, stillborn, or die shortly after birth. They may also have no symptoms. They may have a rash on the palms of their hands or the soles of their feet. They may eventually develop deafness, teeth deformity, or the bridge of their nose may collapse.   

Talk to your doctor for more information about syphilis. Visit them for testing and treatment if you think you may have been exposed to this STD.

WebMD Medical Reference

Sources

SOURCES:

Centers for Disease Control and Prevention: "Syphilis - CDC Fact Sheet."

Lab Tests Online: "Syphilis Tests."

Mayo Clinic: "Syphilis."

Neurology Clinical Practice: "Diagnostic tests for syphilis."

NHS inform: “Syphilis.”

UCSF Health: "FTA-ABS blood test."

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