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POSITIVE ELISA/NEGATIVE WB
Nov 4, 2001

I'm a male who recently had an ELISA test come back positive with the follow-on Western Blot negative. Minimum five months after sex with a woman of probably (but not certainly) negative status, and that sex was with a condom - no breaking or any other problem noted. Can you tell me: (1) what are chances that ELISA will show a real HIV infection and WB taken on same sample not show it? (2) What fraction of ELISA tests are false positives? (3) What can cause a false positive in most recent generation of ELISA? (4) Should I get a follow-on test in a few months based on one piece of info I read that ELISA will detect infection before WB will? Thanks!

Response from Mr. Kull

It's important to look at the bigger picture, not the individual pieces, in the entire testing process: your overall result was negative. Don't get wrapped up in numbers (if you can resist). They're not important in your case.

The ELISA test is considered a screening assay and is not used to confirm the presence of HIV antibodies. The ELISA is most useful in ruling out the presence of HIV antibodies, due to its superb sensitivity. The test lacks the necessary specificity to confirm infection, so there will be higher rate of false positives than one would like. Why ELISA tests are falsely positive is not usually clear.

This is why the Western Blot, considered a confirmatory assay, is used after a positive/reactive ELISA. The Western Blot is highly specific, meaning that it is highly unlikely to be falsely positive. The Western Blot is the gold standard of confirmatory assays.

You had a negative antibody test result five months after a low-risk exposure. There is no need for further testing.

See http://hivinsite.ucsf.edu/InSite.jsp?page=kb-02-02-01 for more on technical details of antibody assays.

RMK



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