I don't follow the test "generations" especially closely. But even the earliest ("first generation") HIV antibody tests used in the 1980s generally are positive within 45 days. You can be over 95% confident in the results, maybe 99%.
Good luck-- HHH, MD
I believe that with either a third or fourth generation test, negative at 45 days is just as accurate for both.
When I was worried a few years ago I had been infected, I remember reading that if the immune system of an individual is compromised prior to an HIV exposure (by another disease, chemotherapy) that these were the people that it took a longer time to convert to positive. I think this may be where the original 3 to 6 month guideline came from. I am sure there have been a few individuals who were late for no apparent other reason, just freak occurrences.
Also like has been said before on this forum the tests are better now. I found this document from the CDC given the average window of 25 days (CI 9-41 days), and this was printed back in 1996.
Some labs in the US still use the 1st gen test but this test is much improved over the 1980's since the chemicals and methods used are much more advanced. Home access uses 1st gen but even with that test its only 5-7 days slower than a 3rd gen.
The generation is really irrelevant if you wait at least 6 weeks to test. If you are just in the process of converting, then it might make a difference but if you are ok at 45 days, then you are probably ok. You can always test again at 13 weeks if you want to follow official cdc guidelines, but you will likely be neg
A 4th generation HIV test is a procedure using a 3rd generation ELISA (or EIA) antibody test combined with a P24 antigen test. This allows for detection of HIV about 5 to 7 days sooner than by a 3rd generation test by detecting the P24 antigen.
As carnageofthepast notes, test generation is only an issue when you are very near the time of seroconversion. The average time to seroconversion is about 21 days, so you are well past that time period. Your negative test at 45 days is highly indicative that you are HIV negative.