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HIV Duo Test
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This forum is limited to prevention of HIV and to safe sex in general. All questions will be answered by H. Hunter Handsfield, M.D. or Edward W Hook, MD.

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HIV Duo Test

Dear Dr Handsfield/Dr. Hook,

I am a resident in the United Kingdom. I had a possible HIV exposure on the 18th of September 2010?

The UK Guidelines on HIV testing from September 2008 state that a 4th generation test - the HIV DUO - should be used as the test of choice across the UK. I don't trust antibody testing in the UK because I recent times it has known to have somewhat of a higher-than-before false positive rate (besides I don't want to go through the rude shock of being falsely tested positive of HIV).

My questions are these:

(1) How long does the p24 antigen remain detectable to appear on an HIV test?

(2) Are there any medications that would make the p24 antigen undetectable? If yes, what are they please?

(3) I know that it would be a waste of time, money and emotional energy to test this frequently, but despite all of this I aim to get HIV Duo tests done at 5 months and 10 months after my sexual encounter with the escort that I had. Would the p24 antigen remain detectable right throught those 10 months to appear on an HIV test?

(4) In the HIV Duo test will the p24 antigen only detect HIV-1 or will it also detect HIV-2? If the p24 antigen also detects HIV-2, will the p24 Antigen of HIV-2 remain detectable right throughout those 10 months I mentioned? Also if the p24 antigen can detect HIV-2 throughout those 10 months, are there any medications, if taken, that can make the p24 antigen of HIV-2 undetectable? If yes, what are they please?

(5) I am aware that no biological tests can be a 100% accurate due to a statistical impossibility. The closest it can get is 99.99% recurring. At 6 months (or more) post possible exposure an HIV test would have reached it's statistical maximum of 99.99%. Even though it isn't 100% accurate, is it possible to have a false negative at that stage? Also, if the test is negative (God Forbid) is the result fully conclusive even though it isn't a 100% accurate?

Thank you!
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239123_tn?1267651214
Welcome to the forum.  I'll try to help.

I don't share your lack of confidence in antibody testing in the UK and do not believe it is possible to generalize test reliability differences between UK, Europe, North America.  And my understanding is that DUO test procedures is pretty much foolproof; the specimen goes into a machine and the results for both antibody and p24 antigen then are read out.  Performance of labs in the UK on other antibody tests is irrelevant to DUO reliability.

Also, your question suggests you may misunderstand an aspect of the DUO test. It is HIV antibody itself that clears p24 from the blood.  Therefore, how long the p24 antigen component of the test remains positive is irrelevant; when testing is done 4+ weeks after acquiring HIV, every infected person has p24, antibody (or, briefly, both) -- and thus a positive DUO test.  This is why DUO is now recommended as the HIV test of choice in the UK, and why I hope the same recommendation will be forthcoming soon in the US.

To your specific questions:

1) For the reason above, p24 declines as antibody rises, generally in the 3-6 week range.  Both may sometimes be detectable out to a couple of months, however; I'm not certain.

2) No medicines have any known effect on p24 detection.  Very potent immunosuppressive drugs or chemotherapy may delay antibody production, but even this effect is theoretical, with few if any actual known instances.

3) I agree you are planning on much more testing than you need.  It is not possible to have HIV and have a negaitve DUO test beyond 4 weeks.

4) The DUO test is designed to detect both HIV-1 and HIV-2.  I don't know whether the time course of p24 and antibody might be different for the two virus types, but I doubt it.

5) On a theoretical level, it is true no test can be proved to be 100% accurate.  However, this is a moot point.  Here is why:  From my understanding, we can estimate a 1 in 1,000 chance your escort partner had HIV.  For a single episode of unprotected vaginal sex, we can estimate an average female to male transmission risk of 1 in 1,000.  So before you are even tested, the odds you caught HIV are in the range of 1 in a million (0.001 x 0.001).  If you have a DUO test at 4 weeks, and if we assume it's "only" 99% sensitive by that time, then the negative result means the chance you are infected becomes 1 in 100 million.  In other words, zero for all practical purposes.  At 99.99%, it's 1 in 10 billion.  If that doesn't satisfy you, obviously there is no test that will do so and you're just going to have to live with that level of uncertainty.

So my advice is to stop worrying about (or researching) the performance of the DUO or any other test.  Just follow the stnadard recommendations in your country, i.e. have a single test at 4+ weeks after your exposure, then move on.

I hope this helps put things in perspective.  Good luck--  HHH, MD
3 Comments
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277836_tn?1359669774
First of all sorry to jump on this thread but I figured I could ask a question.. Do you see the CDC eventually changing there recomendations on the testing time for the window period?? I have seen a few threads where you hint at testing periods with todays modern test...

I hope you have been well?
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239123_tn?1267651214
I don't know what CDC might say, or when, about the DUO test.  For antibody-only tests, I don't anticipate a change in CDC advice in the near future.  This thread explains why:

http://www.medhelp.org/posts/HIV-Prevention/-A-Question-on-Testing/show/1347755
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This Forum's Experts
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H. Hunter Handsfield, M.D.Blank
University of Washington
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Edward W Hook, MDBlank
University of Alabama at Birmingham
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