[Question #1903] Isentress as PEP & Antacids - Efficecy?

53 months ago
Doctors,

In the very early early morning hours of March 5th, after a heavy night of drinking I solicited a MTF trans commercial sex worker from the Internet. After some foreplay, she told me that she was sure she was HIV negative and has been taking Truvada as PrEP everyday for the last year. I wasn't thinking clearly so after hearing that we engaged in unprotected anal sex, I was the top and ejaculated in her anus.

Once I came to my senses a few hours later I immediately panicked and went to the urgent care. I was prescribed Truvada and Isentress - I took my first dose of both 10 hours after the possible exposure. Two days later I got back in touch with the CSW and asked her to get tested again. She agreed and went to a clinic. She sent me a picture of the results on March 8th and it was negative, the clinic had used the Alere 1/2 Antibody test. While that brought some relief, I decided to see an infectious disease specialist who recommended that I finish the PEP.

I'm now on day 23, and haven't missed a dose of either. However I have been taking Zantac 150, Gaviscon and Tums throughout the treatment as I tend to get very bad heartburn. Recently I came across a few sites (including thebody.com) which said that antacids can impact the efficacy of Insetress. Once I read that I started to panic a bit again. 

Assuming the worst, and she was either lying or has an acute infection that isn't showing yet how effective would the treatment I am on be, knowing that I have been taking medication throughout the course of PEP that can impact the efficacy? Also, I know I will need to test at 3 and even 6 months (if all is negative to that point), but if I were to test negative at 6 weeks post exposure (2 weeks post PEP) how reassuring would that be? As I'm sure you can imagine I am very worried about the position I've put myself in. 

Thanks for your help! 
H. Hunter Handsfield, MD
H. Hunter Handsfield, MD
53 months ago
Welcome to the forum. Thanks for your question.

We prefer not to play worst case scenario games. It really isn't useful, in my opinion, to assume your partner was lying, and it's also unreasonable to suspect she has early, transmissible HIV infection despite taking Truvada as PrEP. Indeed, in this circumstance I would have recommended against PEP for you. In addition, although antacids containing magnesium or aluminum can interfere with absorption of raltegrevir (Insetress), reducing its effectiveness, it doesn't completely abort the effect of the drug; and Truvada alone is highly effective as PEP. All things considered -- the low chance your partner had HIV, the relatively low chance of HIV transmission for the insertive partner in anal sex (average once for every 500 exposures, if the anal partner has HIV), and the effectiveness of PEP, even without raltegrevir -- the chance you are infected is zero for all practial purposes. That said, it would be wise for you to recheck with the doctor who pescribed PEP and see if perhaps they would modify (e.g. extend?) your PEP treatment. Probably not, but we cannot give specific treatment advice; that's the role of the doctor you are seeing.

I agree with your comments about follow-up testing. There are no data on the need for delayed testing after PEP, but most experts play it safe and recommend testing at 3 or even 6 months. Here too, follow your doctor's advice. At the same time, a negative 4th generation (antigen-antibody) test at 6 weeks, as you suggest, would be highly reassuring and maybe even conclusive. One of the downsides of PEP, which in my experience is too often ignored, is that the need for delayed testing prolongs anxieity by requiring delayed testing. Without it, a conclusive test can be done 4 weeks after exposure, instead of 3-6 months. This is one of the reasons I would have counseled against PEP for this particular exposure.

I hope this information is helpful. Let me know if anything isn't clear.

HHH, MD

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52 months ago
Dr. 

I completed the course of PEP as prescribed, and went in for follow up testing 2 weeks and 2 days after completing the medication. The doctor ordered an RNA test which just came back as negative. I know I will need to retest but how confident should I be in this result?

Thanks!! 
H. Hunter Handsfield, MD
H. Hunter Handsfield, MD
52 months ago
Thanks for the follow-up.

You can be very confident. Even without PEP, I might guesstimate the chance you caught HIV as 1 in 100,000. If PEP is 90% effective, it reduced those odds to 1 in a million. And if we estimate that 90% of new infectious would be detected by an RNA test 2 weeks after completing PEP, the chance you have HIV is now around 1 in 10 million -- low enough to be consiered zero for practical purposes.

As noted above, the schedule of HIV testing after PEP isn't really known very well, but I imagine your doctor will recommend additional tests for at least a few weeks (see my initial reply above). You can continue to expect negative results.

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