Dear Alice,

I recently had a routine HIV screening done for prenatal labs. It was positive. Thank GOD, western blot came out to be negative. My OB is reassuring me not to worry, that false-positives sometimes come out in pregnancy, and as long as the western blot is negative — I'm negative. Could you explain a little more in detail as to why and if you agree with my doctor? I'm at very little risk otherwise and had a negative HIV test about five years ago and am with the same person (my husband).

Dear Reader,

Getting a false positive HIV test result can be stressful, especially when you are expecting. Based on what your say, you can have confidence in your doc and the negative result of your second test. The Western Blot test is a confirmatory test that is definitive, provided you were not infected during the three-months before your exam. For more info on the window period, see How long does an HIV test take? and HIV Transmission: When does it show up on a blood test? If the Western Blot is negative, you do not have any HIV antibodies in your blood. If this still concerns you, you can always have another HIV test.

When your blood is tested for HIV, the lab first performs what is known as an EIA or ELISA (enzyme-linked immunosorbent assay). A negative ELISA means that no antibodies were found in the blood and that the person is HIV-negative. A positive result is confirmed with a second ELISA. If the second ELISA is positive, a Western Blot must be done to ensure that the antibodies detected in the ELISA test are really HIV antibodies. The Western Blot test can come back positive, negative, or inconclusive. It seems your provider followed standard procedure — after you had a seemingly positive ELISA, s/he used the Western Blot to dig a little deeper and get a more reliable answer. The negative result from the Western Blot is definitive, and you needn't worry.

About 0.2 percent of ELISA tests give positive results that are then proven false by the Western Blot test. An ELISA test can be falsely positive for several reasons, including a patient's autoimmune disease, multiple pregnancies, blood transfusions, liver diseases, parental substance abuse, hemodialysis, or vaccinations for Hepatitis B, rabies, or influenza. Any of these conditions can stimulate the immune system to produce antibodies that cross-react with HIV antigens and produce a false positive.

Many health clinics are now using rapid HIV tests instead of the ELISA test. Rather than sending a blood sample to a lab, rapid testing is done on the spot and results are available in about 20 minutes. Several rapid HIV tests have been developed that can test saliva or blood from a finger prick. The accuracy of rapid tests is similar to ELISA, and positive results still need to be confirmed by Western Blot.

According to the Centers for Disease Control (CDC), ELISA and the Western Blot test are extremely accurate when used in combination. There is a very, very small chance that a Western Blot result is incorrect. False negatives usually happen when people test during the window period. False positives from a Western Blot most often result from volunteers in HIV vaccine studies, clerical error, contaminated specimens, or misinterpretation of the results.

So all that said, you can trust your doc and shift your focus on other things. It's good to know that you are looking out for yourself and your baby to be.


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