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Dr. Andrew Rynne

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What causes false positive HIV test results?

Answered by
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Dr. Sheetal Verma

Infectious Diseases Specialist

Practicing since :2001

Answered : 635 Questions

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Posted on Fri, 6 Nov 2015 in HIV and AIDS
Question: Hi,
I e-mailed Dr. XXXXXXX Bhavsar 3 weeks ago here on HC Magic about my worries about false positive HIV tests, particularly some research I found saying that flu shots and allergy shots could increase the risk of false positives. At the time, I was too afraid to get the STD testing done. So I went ahead and got a flu shot early Oct and my allergy shots 10/9. However, I still would feel relieved if I did the test and it came back negative. How likely are flu and allergy shots to cause a false positive – either on the ELISA or more specialized tests? How long would you recommend waiting ideally after the flu and allergy shots? I only get the allergy shots once a month and I could even push that a little longer if needed. Or is there almost no risk of false positives?

Note that I don’t have any actual STD symptoms. I just have substantial health anxieties and for a long time have wanted to get this testing done but been extremely afraid to do it. Would the antigen and nucleic acid tests be vulnerable to false positives from flu or allergy shots or is it just the screening antibody tests like the Elisa? Although I could get the test right away, my history is low risk. I’ve had only a few instances of heterosexual protected sex (5 to 10 years ago) and one many (about 15 years ago) that was unprotected. I haven’t shared needles or had a blood transfusion or had any other risks either. This is mostly just an irrational fear on my part but I want to at some point do the tests so I can feel better. It’s hard to explain but I’ve started dating and I was hoping to be able to say to a new partner that my test results are all negative and to find out if hers are. Thanks for your help.
doctor
Answered by Dr. Sheetal Verma 1 hour later
Brief Answer:
Dont be scared get the tests done.

Detailed Answer:
Hi
Thanks for your query at the HCM.

Firstly I would appreciate to your medical knowledge regarding the situation.There are few reports in medical literature and research that found flu vaccination can cause some degree of nonspecific cross-reacting antibodies with HIV-antibody tests. But these cross-reacting antibodies cause "indeterminate" rather than "false positive" HIV tests in rapid test, ELISA and western blotting. The risk of these indeterminate tests is quite low. In these situations it is recommended to get HIV PCR test done. I would suggest you to get the HIV test after 6-7 weeks and 4 th generation ELISA test after 12 weeks. But yes if you have taken shot the test at 6 months post exposure will be considered conclusive.

Stay healthy
Hope I answered your query
Rating and review will be appreciated
Dr Sheetal Verma
Above answer was peer-reviewed by : Dr. Chakravarthy Mazumdar
doctor
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Follow up: Dr. Sheetal Verma 19 minutes later
OK - Thanks. Just to clarify: Were you saying that you would wait 6 to 7 weeks after the shots for the PCR test and then a little longer for the Elisa (rather than doing it now)? Would the PCR be very unlikely to come back falsely positive or indeterminate even now rather than waiting 6 to 7 weeks?
doctor
Answered by Dr. Sheetal Verma 2 hours later
Brief Answer:
PCR 6-7 weeks post exposure.

Detailed Answer:
Hi

I suggested you to take the PCR after 6-7 weeks post exposure and not post shot. It will not be affected by the shot as it detects the HIV viral DNA which does not cross reacts with the other antigens or vaccines.

Take care
Above answer was peer-reviewed by : Dr. Chakravarthy Mazumdar
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