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p24 antigen window period
Thanks, medhelp doctors, for a fantastic service.
You must get tired of this kind of query, but I'd like to ask about the p24 antigen part of the HIV Duo test. I've read that modern antibody tests are pretty much conclusive at 8 weeks and that Duo tests are 99.8 per cent accurate at 28 days. A specificity like this it would suggest that p24 is always detectable by 28 days, except in extraordinarily rare cases. But I've read in different sources that whilst p24 reduces the window period for HIV testing, it's usually by around 10 days, so if I developed antibodies say around day 50, would that mean a negative Duo result on day 28 could be a false negative?
The reason I ask is that I had a negative Duo on day 29 after unfortunately having receptive anal sex with someone I later discovered to be HIV+ (his most recent viral load measurement was around 700), and although we used a condom and there was no ejaculation I'm still very anxious about the result - I guess because unlike many people who think they might have been at risk, I know that the other person was positive.
You must get tired of this kind of query, but I'd like to ask about the p24 antigen part of the HIV Duo test. I've read that modern antibody tests are pretty much conclusive at 8 weeks and that Duo tests are 99.8 per cent accurate at 28 days. A specificity like this it would suggest that p24 is always detectable by 28 days, except in extraordinarily rare cases. But I've read in different sources that whilst p24 reduces the window period for HIV testing, it's usually by around 10 days, so if I developed antibodies say around day 50, would that mean a negative Duo result on day 28 could be a false negative?
The reason I ask is that I had a negative Duo on day 29 after unfortunately having receptive anal sex with someone I later discovered to be HIV+ (his most recent viral load measurement was around 700), and although we used a condom and there was no ejaculation I'm still very anxious about the result - I guess because unlike many people who think they might have been at risk, I know that the other person was positive.
MEDICAL PROFESSIONAL
Welcome to the forum.
First, you had a low risk exposure, both because you used a condom and your partner's HIV viral load is very low, probably too low to transmit even without a condom.
More important, you misunderstand the HIV duo test, i.e. the combination of p24 antigen plus antibody testing. Because the antigen persists only a while, and then antibody appears, you fear there could be a time after p24 goes away and before antibody appears when the test miss an HIV infection.
It doesn't work that way. It is the antibody that clears p24 out of the blood -- that is, the p24 part of the test doesn't become negative until antibody appears. Both are detectable for a few days to weeks, but then the p24 part of the test becomes negative. If for some reason you didn't produce antibody until day 50, then p24 would still be in your blood and detected by the duo test.
Therefore, everybody with a new HIV infection has either p24 or antibody in the blood (or both) at any time 4 weeks or more after being infected and a negative duo test at 28 days is always conclusive. It doesn't matter how high or low the risk was, or what symptoms may be present: the test result rules!
So all is well. You can move on with no worries about it.
I hope this has helped. Best wishes-- HHH, MD
First, you had a low risk exposure, both because you used a condom and your partner's HIV viral load is very low, probably too low to transmit even without a condom.
More important, you misunderstand the HIV duo test, i.e. the combination of p24 antigen plus antibody testing. Because the antigen persists only a while, and then antibody appears, you fear there could be a time after p24 goes away and before antibody appears when the test miss an HIV infection.
It doesn't work that way. It is the antibody that clears p24 out of the blood -- that is, the p24 part of the test doesn't become negative until antibody appears. Both are detectable for a few days to weeks, but then the p24 part of the test becomes negative. If for some reason you didn't produce antibody until day 50, then p24 would still be in your blood and detected by the duo test.
Therefore, everybody with a new HIV infection has either p24 or antibody in the blood (or both) at any time 4 weeks or more after being infected and a negative duo test at 28 days is always conclusive. It doesn't matter how high or low the risk was, or what symptoms may be present: the test result rules!
So all is well. You can move on with no worries about it.
I hope this has helped. Best wishes-- HHH, MD
Dr Handsfield, thanks for your reply to the user who commented on my original query on p24 testing
I wonder if he's referring to this link, or some other source:
http://emedicine.medscape.com/article/1982802-overview#aw2aab6b5
in which there's reference (near the end) to a second diagnostic window period:
Current evidence has shown a second diagnostic window when using fourth-generation assays. In a case of acute infection, 2 fourth-generation assays were compared with 2 third-generation assays and 1 HIV antigen assay. Reactive results from the fourth-generation assays became negative during a second diagnostic window when HIV-specific antibodies were absent and p24 antigen concentrations declined below the test limits.[16] Similar finding have been reported in other case reports.[17]
To be honest, I wish the other user had asked his own question rather than posted on the original thread, as it made me feel anxious again. But I have far more faith in your opinion and expertise, after many years in the field.
Thanks again for your comments and original advice.
I wonder if he's referring to this link, or some other source:
http://emedicine.medscape.com/article/1982802-overview#aw2aab6b5
in which there's reference (near the end) to a second diagnostic window period:
Current evidence has shown a second diagnostic window when using fourth-generation assays. In a case of acute infection, 2 fourth-generation assays were compared with 2 third-generation assays and 1 HIV antigen assay. Reactive results from the fourth-generation assays became negative during a second diagnostic window when HIV-specific antibodies were absent and p24 antigen concentrations declined below the test limits.[16] Similar finding have been reported in other case reports.[17]
To be honest, I wish the other user had asked his own question rather than posted on the original thread, as it made me feel anxious again. But I have far more faith in your opinion and expertise, after many years in the field.
Thanks again for your comments and original advice.
MEDICAL PROFESSIONAL
Follow-up questions by users other than the original questioner usually are deleted without reply, but this is obviously a pertinent issue. I have to assume it's an older article -- and "scientific journal" doesn't, by itself, assure reliability. The clear consensus of the experts is that indeed the duo test at 4 weeks is "just as accurate as it would be in 6, 7,8, or even 11 weeks". Can you provide the exact source of the statement?
Thanks. This has really cleared up my misunderstanding and finally dealt with my anxiety.
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