The presence of IgM antibodies to human immunodeficiency virus 1 (HIV-1) was investigated in blood donor sera which were indeterminate in anti-HIV-1 IgG Western blot testing. In 7 of 173 instances out of approximately 1,000,000 blood donation sera with an isolated anti-p24 IgG produced an anti-gp41-45 IgM immunoblot reaction. Applying polymerase chain reaction (PCR) to 29 indeterminate samples out of approximately 125,000 blood donations it was found that 2 of them were IgM-positive and also contained HIV-l-specific DNA sequences. Eleven months later 1 of these 2 donors was retested and found IgM and PCR negative.

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