QUESTION
I had sex with a partner who is HIV positive. I used a condom when I topped him. I noticed my condom slipped off after I pulled my penis from his ass. I am really concerned that I could have gotten HIV. Could I have if I was the top? He has a really low viral blood count.

ANSWER
You were wise to be using a condom for anal intercourse with your partner regardless of your knowledge of his HIV status. Continue to do that and you'll keep your risk of HIV and STDs very low.

Since you had a condom on during the intercourse itself, what you described was not really a risky situation.

As to your other questions, yes, if you had not been using a condom, it is possible for an HIV positive bottom to transmit the virus to an HIV negative top even though it is a much less risky behavior than the other way around. An HIV-negative bottom has about a 1 in 50 chance of getting HIV infected after one act of intercourse with an infected top.

As far as viral load, research has shown that the viral load in blood correlates to levels in semen and vaginal secretions. However, that correlation is only about 70% and it has not been proven what level of virus is needed for transmission. Even an undetectable viral load doesn't mean an infected person will not transmit HIV to an uninfected partner. Therefore it is very important that you continue to use condoms no matter your partner's viral load. In addition, a person's viral load can change, so that viral load a month ago may not be the viral load the day you had sex.

To your health,
Dr. K