False Positive ELISA

[worriedkarlie]

I am a 26 year old white female from the US.  Have never used IV drugs.  I have only had sex/sexual relations in the context of a committed relationship (I have been in three such relationships = have had intercourse/sexual relations with three men; all three are heterosexual, white, from the US).  In my last relationship I took a routine STD screening as part of a yearly physical.  I have never had any STDs/abnormal paps in the past.  To my shock, my ELISA was positive.  Luckily my Western blot was negative.  This was on May 11.  My physician was not concerned.  She sort of brushed it off, told me it happens, she's seen it before, and that I should take a blood test 6 mos down the line as sort of a technicality.  This made me feel much better, but I was a little freaked out because my boyfriend and I were not exactly the best about using condoms (he never came inside of me, though).  My boyfriend and I have since broken up.  We last had sex on June 23.  I have not had any sexual encounters since then.  Recently I started freaking out -- thinking, "What if I'm seroconverting?"  I took a rapid test (blood sample) on September 30.  It was negative.  This would have been about 4 1/2 months since my positive ELISA/negative Western Blot and about 14 weeks since my last possible exposure.  I guess I'll go back at the 6 month mark just to be sure, but that false positive really wigged me out.  I keep thinking maybe the rapid test was a false negative!  I haven't been an overly anxious person in the past, but this triggered a nutso anxious response in me!  I found myself thinking, "Maybe the rapid test was wrong!  Maybe they screwed up my test with someone else's.  Maybe the nurse used dirty needles for the recent blood draw and NOW I'm infected."  I know this is totally crazy, but this false positive ELISA back in May has completely shaken my confidence in testing. Thanks for your time!

[Edward W Hook, MD]

Welcome to the Forum.  A false positive test result can be very disconcerting and thus it is not surprising to me to hear of the thoughts that are running through your head.  Let me assure you that you can believe your combination of test results you describe and really do not need further testing.  You do not have HIV!.

False positives occur rarely but they do happen somewhere between 1 in 1000 and 1 in 10,000 tests. In some cases the false positive tests are transient and go away and in others they are persistent.  In the instances of transient false positives we tend to ascribe the FPs to antibodies to some other process (such as a non-HIV viral infection or a medication or just bad luck) which then go away with the passage of time.  In other people there are antibodies in their blood which, for some unknown (and unlucky) reason cross-react with the proteins used in HIV blood tests.  Since your follow-up rapid test was negative your FP may have been transient or it may have been directed at a protein other than the one included in the rapid test.  Either way, the combination of a negative Western blot, and a negative rapid test more than 17 weeks following your prior test in clear indication that you do not have HIV.  I urge you not to worry and am confident that you do not need further testing at this time.  

I hope this comment is helpful to you.   EWH

[Edward W Hook, MD]

Absolutely.  False positves are far, far more common than false negatives.  

Unfortunately there is no way to predict whether or not your will experience another FP in the future.  Sorry.  EWH

[worriedkarlie]

Thank you so much for your quick response!  It is reassuring and makes sense.  Am I correct in assuming that a false positive ELISA is far more common than a false negative ELISA/rapid test?  (i.e., why trust a negative test when a positive test proved false in my case?)

Also, as my false positive situation seems to be transient, am I likely to have a repeat "transient" experience or do those generally seem to be one-time anamolies?  Or is this something we do not know?

Thanks again!