False Positive

Question

Dr. Frascino,

Hi. My partner and I have been together for two years. He has been HIV positive for 13 years and his viral load is undetectable. He remains exceptionally healthy. We do not practice safe oral sex. For precautionary measures, I have 3-4 HIV tests per year. In May 2007, my Elisa test came back positive and the Western Blot negative. My PCP referred me to an infectious disease specialist because of the situation with my partner being positive and me being negative. In July 2007 the tests with the specialist returned the same results. My question is why? Over the past two years, four out of six tests returned a negative Elisa, while my last two returned a positive Elisa with an negative Western Blot. The specialist stated the virus may be in the early development stages, but again, unlikely because my viral load is at 400 or undetectable. What are the reasons for the positive Elisa with a negative Western Blot?

Thank you for your time.

Answer

Hello,

A reactive ELISA and negative Western Blot (WB) is considered a negative HIV-screening test. The ELISA test is set to pick up as many true positives as possible (very sensitive test). In doing so it also picks up some "false positives." That's why a reactive ELISA must always be followed-up with a confirmatory more specific test, such as a Western Blot or immunofluorescence assay (IFA), to confirm the reactive ELISA before the test (combined ELISA and WB) can be termed "positive." There are many reasons why ELISAs can show "false positive" results, including cross-reacting nonspecific antibodies that trigger a positive result, but that have nothing to do with HIV.

If your results are in dispute or should your Western Blot become indeterminate, which can also happen with nonspecific cross-reacting antibodies, I would recommend you get a DNA PCR (rather than RNA viral load).

Considering your two reactive ELISAs had negative WBs and were run two months apart, I strongly doubt you are seroconverting.

Dr. Bob