All my life I have had false-positive tests for TB and syphilis. Further testing always showed I was free of disease. I am now eight weeks pregnant. Not only was my syphilis RPR test positive, but so was the follow-up FTA test. My husband and I live on a dairy farm and have a lifestyle as distant as possible from any of the ways of transmitting syphilis that I know of. However, the lab tech says the FTA test is 100 percent accurate. Is this true? Is there some way to get syphilis other than extramarital relations, drug use or blood transfusions?

E.

This is a very difficult topic. Let me start by answering your last question. The bacterium that causes syphilis can be acquired through sexual intercourse (by far the most common method), kissing, passage from mother to fetus through the placenta, blood transfusion or accidental contact with an infected lesion. I am not sure that living on a dairy farm means you are unlikely to have syphilis, but if you and your husband had no prior sexual contacts, blood transfusions or other exposures, then I doubt that you really have the disease.

If you indeed had no possible exposure to syphilis, then why did you test positive? It doesn't matter what the test is, there is always a chance of a false result. NO TEST IS 100 PERCENT ACCURATE. There can always be a laboratory error or a mistaken report, for a variety of reasons.

In general, we can gauge the likelihood that a test result is incorrect by looking at the test's "sensitivity" and "specificity." The sensitivity refers to the probability that a person with the disease will test positive. If the sensitivity for the RPR (rapid plasma reagin) test for syphilis is 93 percent, then when 100 people with syphilis are tested, 93 will get a positive result. The other seven will receive a falsely negative result. The specificity is the chance that someone without the disease will test negative. If the RPR test is 88 percent specific, then when 100 healthy people without syphilis are tested, 88 will test negative, and 12 will have a false-positive result.

These numbers do not help us much in cases like yours. We want to know what the chances are that you have the disease after you tested positive by both the RPR and FTA (fluorescent treponemal antibody) tests. What we want to know is called the "positive predictive value" (PPV) of the pair of tests. In your case, the PPV depends in part on the probability that a person with your risk factors, symptoms (or lack of them) and medical history would have the disease. If these characteristics indicate a low probability of disease, then the PPV will also be low. To put it another way, if you test a large group of nuns for syphilis using these tests, you can expect that most of the positive results will be false. Given what you have told me, it is likely that your FTA and RPR results are falsely positive. It is true that the FTA is highly accurate in showing that a person was once infected with the syphilis organism, but it is not 100 percent accurate.

However, this does not mean that I would not consider treating you for syphilis. If there is a chance that you have the infection, it may be wise to treat you in order to prevent the very small risk of your baby becoming infected. It is not common for a fetus to be infected before the fourth month, so even if you do have syphilis, the baby is probably still okay. Syphilis is treated with penicillin, which is considered safe in pregnancy.