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General Medical Questions
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Question : I give blood, but I keep getting a false positive for syphilis. After additional testing it comes back negative every time. What could cause this to happen?
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The Trusted Source
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Howard LeWine, M.D.

Robert H. Shmerling, M.D. is associate physician at Beth Israel Deaconess Medical Center and associate professor at Harvard Medical School. He has been a practicing rheumatologist for over 20 years at Beth Israel Deaconess Medical Center. He is an active teacher in the Internal Medicine Residency Program, serving as the Robinson Firm Chief. He is also a teacher in the Rheumatology Fellowship Program.

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February 15, 2011
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A:

A false-positive test for syphilis means that the test is abnormal (or positive) despite the absence of syphilis. Another test (called the FTA) can rule out or confirm syphilis if the first screening test (called the RPR) is positive.

The most common reason for a false-positive is that a person produces antibodies that "fool" the syphilis test. This may occur because the antibodies are similar to the syphilis antibody or because they interfere with chemical reactions needed to perform the test for syphilis.

In many cases, the antibodies are produced because of another illness. Examples include an infection (such as chickenpox, Lyme disease or certain types of pneumonia) or an autoimmune disease (such as lupus).

However, some people with a false-positive test for syphilis have it for no identifiable reason.

Because you know your test is repeatedly positive, you do not need the routine screening test (the RPR) in the future. If syphilis testing is recommended for you, it should only include the FTA test.

Talk to your doctor about your false-positive test results. Together you can make a plan to avoid having this RPR and decide whether other evaluation is needed.

In most cases, it's not helpful to conduct an exhaustive search to find a reason for your false-positive syphilis test. However, focused testing may be needed for people with a false-positive test for syphilis who also have antibodies (called "antiphospholipid antibodies") that are associated with abnormal blood clotting. People with these antibodies and blood clots usually receive lifelong treatment with blood thinners. So, if you've had abnormal blood clots and a false-positive test for syphilis, testing for antiphospholipid antibodies may be appropriate.

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