Your follow-up comment below indicates your last test was 7 months after exposure. You can be 100% certain you don't have HIV, regardless of the initial false positive result. I'm sure that was disconcerting, but the repeat and confirmatory testing always tell the truth. I'm not an expert on the frequency of false positive ELISA results with modern testing, but it is rare, less than 1 in several hundred, maybe around 1 in 1000. In any case, you can be secure in the knowledge you don't have HIV.
I wonder about how the blood bank went about telling you about the initial false positive result. Of course you would want to know about it, in the event you have future HIV testing--especially a rapid test, when you might be given a result before confirmatory testing. On the other hand, if/when that happens, the provide has a responsibilty to present the information in the most reassuring manner possible. (But perhaps they did that. I'm not criticizing the blood bank, not knowing whether you might have had the same reaction regardless of how the results were presented.)
Good luck-- HHH, MD
I'm sorry, I forgot to mention how long after the last possible time of infection I was tested. 7 months, and I know this is well past the window period.
Thanks for the reply doc... to answer your wondering.. I received a letter saying more or less: "Your blood donation tested positive for HIV using antibody screening tests but negative using the Western Blot and Nucleic Acid Amplification Tests. You are considered not infected with HIV."
It's good they didn't tell me the positive before doing the comfirmatory tests, but the way the did it is not comforting at all to someone who prior to this had no knowledge of any of these tests. Take care.
That seems an insensitive and inappropriate mode of communication by the blood bank. And the terminology "considered not infected" isn't quite as definitive-sounding as something like "These expanded test results show you are not infected with HIV, despite the initial false-positive test result". I believe they have a responsibility to personally contact such donors, or at a minimum to offer to discuss the results. It's not as if it comes up all that often; at current false positive rates, for most blood centers it probably comes up only a few times a year. Feel free to tell 'em I said so; fine with me if you forward this message to them.
Is there a false-negative?
Yes. F-Negs can occur when testing is carried out too early in the window period. Most antibody tests will turn positive after about 21 days (i'm not a doctor and nor is this a definitive answer). But the vast majority of 4th gen tests are good from about a month and then pretty much bang on after 6 weeks.