One of the last days of December 2021, I had sex with a female CSW for around 75 min. The intercourse was vaginal with protection. At some point, the condom broke in the top and my glans was exposing to the woman´s vaginal secretions for about 5 to 8 minutes. Exactly 29 days after de expose, I got tested for a HIV 4th generation combo and received a negative result. Four weeks after, I decided to see an infectologist. The doctor asked me if I had any STD symptoms, but presenting none of them. Regarding HIV, the doctor told me that it was a low risk and it would´ve been very difficult for me got infected. After 9 weeks of the risky contact, as the doctor indicated, I got testing for Hepatitis B, C and syphilis, all went negative. Additionally, 3 months after my medical consultation, doctor requested new tests, so I will have to repeat  them all and it´s going to be by the end of next May. I don´t understand why I should repeat all of them, is it the first ones are not reliable enough? Is there any odds to got caught an infection that didn´t show up erlier?  I´m confused, why  a couple of years ago a HIV 4th gen. test was considerd deem conclusive at 4 weeks and now is 95-98 % effective? Is it possible that the effectiveness of the test decrease in this years? How could I know if I belong of the small percentage that doesn´t registed the test as positive at the first time? 

With several minutes exposure after condom failure, this should be considered an entirely unprotected sexual exposure. Fortunately, even if your partner has HIV, the chance you would have been infected is around once for every 2,500 exposures, so the odds are strongly in your favor. And the large majority of female sex workers do not have HIV. Further, a negative HIV antigen-antibody (AgAb, 4th generation, combo) test is 98-99% reliable at 4 weeks after exposure. So the chance you have HIV is almost zero -- maybe one chance in a million, tops. Your question isn't clear on your next test:  did you have another negative HIV AgAb test at 9 weeks? If so, that result is 100% conclusive and no more testing is needed. However, if you have not been tested since the one done at 29 days, your doctor is correct to suggest another test at this time. But stay relaxed about it:  as noted, there is under one chance in a million you have HIV, so you definitely can expect a negative result.

The hepatitis B and C tests are conclusive after 6 weeks. However, the syphilis blood test sometimes take as long as 8-12 weeks to become positive. On the other hand, if you have not developed a chancre (the ulcer of primary syphiliis) on your penis within 4 weeks, that is strong evidence you did not catch syphilis. You can expect any further test results to also remain negative. 

As for your final question, the answer is no:  None of these tests becomes less reliable over time. Once positive, all remain positive forever (except syphilis, but when syphilis tests become negative because of treatment or cure by the immune system, it takes many years).

Presumably you also had negative urine tests for gonorrhea and chlamydia, right? If all results are negative so far, you should have resumed sex with your regular partner, if you have one. You can be very confident you have no STI from the exposure last December.

I hope these comments are helpful. Let me know if anything isn't clear.

HHH, MD

"My first negative results and no physical evidence of disease so far is a good sign, right?" YES! Maybe you didn't fully understand my replies above. Your 9-week hepatitis B and C and syphilis tests already are conclusive. Another HIV test can be done at any time now, and also will be conclusive. Probably you should follow your doctor's advice, for reassurance. But in truth, you do not need any testing at 20 weeks, unless you decide to wait until then for a conclusive HIV test result.

The number of people in Mexico doesn't make any difference. Regardless of that number, you can be certain you do not have it.

HIV can be transmitted during a single exposure. However, the chance is low. On average, unprotected vaginal sex with an infected female partner has an average transmission risk of once for every 2,500 exposures (according to the US Centers for Disease Control and Prevention). That is why some spouses of infected persons remain free of HIV even after several years of regular sex together.

To be honest with you, and despite everything, getting tested makes me nervous. For me it was an extremly stressful experience. I remembered  having very hard days, full of fear, anxiety, guilt and shame, I felt bad at myself for getting into trouble in the dumbest way possible. 

But you have already given me several reasons to be optimistic to await for the indicated date to retest. I would like to share with you my future test results.

I really appreciate for your advices Dr. Handsfield. Regards 

Buena suerte y mantente seguro.