How long after initial infection is HIV contagious but the test results still show negative?

Probably from 1-6 mo. This window apparently can vary with the individual seroconversion. The recommended testing of patients is at initial exposure, at 6 weeks, at 3 months, and at 6 months. Many specimens positive with screening elisa are indeterminate on western blot, and so can be tested by immunofluorescence assay (ifa) more than one month later. So it must take up to 6 months to get positive test results.