In British Columbia, which uses the 3rd generation HIV 1 and 2 antibodies test, over 90% of people who have been infected with the virus will have a positive HIV test result if they test 6 weeks after infection. The test is considered conclusive at 12 weeks.
It is unlikely that someone who tests negative at 6 weeks will then test positive at 12 weeks, but if there was a high risk for infection a 12 week follow up is recommended. Your doctor may have felt the risk was low and that the 6-week test was all that was needed; The only way to know for sure would be to ask him/her.
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