1.) What is the accuracy of lab based HIV 4th Generation DUO test at 3 weeks (21 days), 4 weeks (28 days)? Would a test at 25+ days be just as accurate/no measurable difference as a test at 28 days?

2.) Have either one of you seen an HIV 4th Generation Test be negative at or around 4 weeks (slightly before/slightly after 28 days) and later become positive at 6 weeks? I know the CDC has updated the guidelines but I just want to know if personally any one of you have personally seen or heard about it other than with the data published by the CDC?

3.) Now with the data published by the CDC, is it indeed a VERY small percentage of people who don’t test positive between 4 weeks and 6 weeks? Or is it significant enough that a negative at or around 28 days would warrant more testing in your expertise with a good likelihood that the result could become positive?

Now, I do ask because I had a negative slightly before 4 weeks around 26.5 days and would like to resume unprotected intercourse with my regular partner. 

Thank you and have a great day. 

1. The AgAb HIV tests ("4th generation", "duo") are around 80-90% reliable at 3 weeks, ~98% at 4 weeks. There's probably little difference in reliability between 25 and 28 days.

2. I have never seen this happen and to my knowledge, neither has Dr. Hook.

3. To my knowledge, there has never been a documented case of someone with HIV in whom the AgAb test was not positive within 6 weeks (or, more accurately 45 days, which on this forum we round off as 6 weeks). The only exception is when people take anti-HIV drugs as pre- or post-exposure prophylaxis to prevent HIV after a high risk exposure. If PEP/PrEP doesn't work, it can delay the time to positive test results.

Can you safely resume sex with your wife? To help assess that, I would need to know the nature of the exposure you are concerned about. If it was genuinely high risk (e.g. unprotected anal sex with another male, or sex with a known infected partner), I think you should wait until you've had a negative test at 6+ weeks. if lower risk, such as vaginal sex with a new female partner, the chance you caught HIV would be low enough that if somehow I were in your situation, I would resume unprotected sex with my wife -- or might never have stopped sex with her.

So let me know about the exposure if you like. But in the meantime, you can be at least fairly confident, and perhaps virtualy 100% certain, that you don't have HIV.

HHH, MD

My risk was a broken condom vaginal sex with a CSW, unknown status. Shady place but a known massage place.  All other STDs negative.

Regardless, with these details, would protected or unprotected matter in this particular instance along with the negative test at 25 days?

If I put this in personal terms, if somehow I were in your situation, I would not have been tested for HIV and would have continued sex with my wife with no delay at all.

In this situation, with a condom, the chance you caught HIV probably was around 1 in 2.5 million, even before you were tested:  Perhaps 1% chance your partner had it, average transmission risk by vaginal sex 1 in 2500, condom 99% effective: 0.01 x 0.0004 x 0. 01 = 0.00000004, 4 in 10 million = 1 in 2.5 million. Even with a test that might be "only" 90% sensitive, your odds become 1 in 25 million. I hope you agree that's zero for all practical purposes!

Final two questions as I know it’s my last:

1.) Would your assessment change with resuming sex along with the negative test change if the condom broke?

2.) Are you saying my test was “only” 90% reliable? Or was it closer to 98% reliable being it was only a couple days short of 28 days?

I know I’m picking at straws here but appreciate your help.

Thank you and sorry for the error. 

2) No, I wasn't saying your test was only 90% reliable. I used that figure for simpler math in the risk calculation I gave you. 98% is about right. (But if you read and understood my calculations, you're realize it doesn't matter anyway.)

If 98-99% of newly infected people have positive results by 4 weeks, of course "it is indeed...very rare" for anyone to be negative at 4 weeks and become positive at 6 weeks. In fact, I have never had such a patient or even heard of one.

That winds up this thread. I hope the discussion has been helpful. Best wishes and stay safe.