Basically: he says he didn't intentionally dope with someone else's blood. He doesn't know the answer, but he and Coyle speculate that Fuentes was sloppy and a bit of someone else's blood got mixed in with his. This might have happened, for example, when glycol was being mixed with the blood.
The book quotes JV as saying the following, which nails it I think:
"The thing to realize about Fuentes and all these guys is that they're doping doctors for a reason. They're the ones who didn't make it on the conventional path, so they're not the most organized people . . . The deadly mistake that Tyler, Floyd, Roberto, and the rest of them made when they left Postal was to assume they'd find other doctors who were as professional. But when they got there, they found--whoops!--there weren't any others."