suggest that you contact the local transfusion service - the people who arrange the donations locally (particularly as you are in the US.There could be some potential issues with blood infusions that contain high levels of B12 though B12 isn't toxic ... but it may be possible that the blood cold be used to make other blood products.
I suffer from PA and am a blood donor with the Welsh Blood Service. In Wales, the advice is that you can donate as long as you are on maintenance doses (not loading doses) and that you feel fit and well on the day.
All I can recommend is looking into the places where you can donate locally and seeing what they advise you.
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