Why do people with lupus test falsely positive for syphilis?

Johns Hopkins Medicine

In the 1940s, doctors realized that some women with lupus also tested positive for syphilis, a sexually transmitted disease that, if left untreated, can cause significant damage to your heart and nervous system. Further studies indicated that one in five people with lupus had a false-positive syphilis test.

If you have a form of anticardiolipin antibodies in your body, you will test falsely positive for syphilis.

It turns out that if you suffer from lupus, and you test falsely positive for syphilis, you also have a certain kind of antiphospholipid antibody in your blood. An antibody is a special protein your body produces to fight off infections and other invaders. But, if you suffer from lupus, your antibodies actually work against your own body and you produce them at a very fast rate. Antiphospholipid antibodies, which about 50 percent of people with lupus have, make blood clots more likely to occur.

We now know that you can have antiphospholipid antibodies without having a false-positive syphilis test and vice versa. The false-positive test is not associated with an increased risk of blood clots in all medical studies performed in the past, but certain studies suggest that there may be a connection.

The false-positive syphilis test was one of the first three recognized indications of antiphospholipid antibodies. The other two are the lupus anticoagulant and anticardiolipin antibody.

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