Hello,I have been reading a lot about PEP, HIV RNA tests, and the duo tests from CDC and other government websites. I have a lot of questions:

1. How does PEP work in stopping HIV prevention? Why is the threshold set at 72 hours? I know PEP acts just like ART medication that treats current HIV patients but I am not sure how does it differ with ART medications.

2. I went to my doctor and ask whether taking PEP would affect the results for HIV testing. She told me that PEP should not affect the HIV test results, and she recommend me to come back and do combo antibody/antigen tests in 4-6 weeks after my exposure.  I also asked my county's public health department about this. The person told me that I could get reliable results 3 weeks after exposure. However, I have seen some conflicting answers here in this forum. Could you shed more light into this? Is there any data that support why PEP would/would not affect test results?

3. I have also read that it usually takes months for HIV patients to get undetectable viral load from ART. Thus, why would taking PEP for only 28 days delay the test results?

 4. Also, I have read about HIV rna tests from stdcheck.com, which says that it would be around 95% accurate after 9-11 days. How accurate is this statement? Again, how would PEP delay the test results from HIV RNA test if it takes months for HIV patients to get undetectable viral load from ART.  I know that current HIV RNA tests have threshold as low as 20 viral copies/ml. Will this low threshold be affected when I am taking PEP, for example?

If you look at my previous question (QUESTION #6403), I know you told me that I have little to no risk of exposure. However, I know that my anxiety comes from the fact that I was pretty drunk that night and thus I am afraid that I may not remember some stuffs. All the things that I say on my previous question is all that I can remember. 

First, your last question in your follow-up comment:  In the CDC document, Table 2, top row:  HIV AgAb test recommended 3 months after PEP as well as 4-6 weeks.

For your other questions, all are covered in the CDC document you found. It's intended for health professionals, but you should be able to understand and follow most of it.

1. PEP works by the same way ART does (the same drugs are used for PEP and treating established infection). PEP inhibits HIV from entering the cells they normally infect and thus establishing a new infection and/or killing the virus in cells they have already invaded. 72 hours is based on research in animals (moslty chimpanzees) inoculated with HIV then given anti-HIV drugs at various intervals; and observations in exposed persons. Efficacy drops after 24 hours, i.e. more failures after 48 hr and still more out to 72 hr.

2. Your doctor and local public health would seem to be out of sync with CDC and most experts. I stand by the advice and timing described in your other thread and CDC.

3. Preventing infection (whether HIV or anything else) generally takes less drug and less prolonged treatment than it does to cure established infection. If PEP fails, it is logical to start the testing clock at the end of treatment rather than exposure, and to provide an additional margin of safety by adding another two weeks.

4. Most experts would say RNA testing reaches 95% reliability 2-3 weeks after exposure and 9-11 days is too early. Here is the review of HIV test performance that forms the basis of our advice on this forum:  https://www.ncbi.nlm.nih.gov/pubmed/29140890 The same issue of the journal also had an editorial from CDC, endorsing the same recommendations.

As for your exposure, I suspect that no matter how drunk you were, you would recall if you'd had a truly high risk exposure, such as unprotected anal sex with another man. None of this changes my evaluation of that event or my advice in your other thread.

Let me know if anything isn't clear.

HHH, MD

1. Correct me if I am wrong, but isn't table 2 on CDC website mentions 4-6 weeks testing post exposure, not after completing PEP? Is there something that I am missing here? 

2. I am currently contemplating whether to stop taking PEP or not. I have taken it for 8 days. I could not ask the doctor that prescribed PEP for me whether I should continue or not since the doctor works in the ER.  The doctor on the post above is a different doctor. 

On one hand, stopping PEP would give me a much faster test results to end this anxiety and move on with my life. My plan if I stop PEP is to get HIV RNA test 2 weeks after stopping (per your recommendation). 

However, on the other hand, I keep thinking on the "what ifs" part such as if I stop now I might get infected instead.  I acknowledge my anxiety is preventing me from thinking logically and using math to calculate my risk of exposure properly (such as being the "special one"/outliers in the statistics).  I even looked up the statistics for HIV prevalence for Asian women in the US. I plan on going to counseling center this week to help me resolve this anxiety. I apologize if I seem oblivious to the facts that you present.

The table advises testing both at 4-6 weeks and 3 months after exposure. Three months is ~8 weeks after completion of PEP. The table does not imply that testing is conclusive 4-6 weeks after expsoure. A negative result at that time would be reassuring but not conclusive.testing should occur

at 4

–6 weeks and 3 months after exposure to determine if

HIV infection has

occurred.

I talked to a doctor today, and he told me to just continue with PEP since I already started it anyway, which is what I am going to do. 

Also, I figured that my irrational anxiety stems more from emotional response such as guilt and shame from what I did than from objective risk. It's the first time ever I did something like this and even though I managed to be as safe as possible (using condoms for penetrative sex, for example), my guilt and shame overtakes my objectivity towards this matter. Thus, I apologize for asking such repetitive questions in the past two threads and thank you for your simple and science-based explanations.

One more thing, based on my case, since you say that there is practically zero risk of HIV, I don't need to avoid having protective sex completely right?

Interesting philosophical question: from a medical/risk perspective, it is certainly safe to have sex. OTOH, if you're taking PEP to prevent HIV to which you may have been exposed, it can be argued that there's a disconnect in having sex. If you do it, I think your partner should be aware of the situation. Even with no risk, if you were that partner in this situation, wouldn't you want to know?

That completes the two follow-up exchanges with each question and so ends this thread. I'm glad to have been of help. But please note the forum doesn't accept repeated anxiet-driven questions on the same topic. This being your second, it will have to be your last about this exposure, PEP, etc. OK?

Take care and stay safe.