I've posted before about my serum B12 levels being over 2000 without supplementing. I was told that could mean a functional deficiency. I've also read that in several places. I've had two homocysteine tests and a methylmalonic acid test. They were both negative for a B12 deficiency. I've also had a CBC done twice and my MCV was normal. I know a high level of B12 can mean you have a lot circulating in your blood but not in your cells. Is it possible those tests might miss a deficiency since there is a lot of circulating B12? It was my,understanding a true deficiency would be detected with a homocysteine test or a MMA test for sure because of the sensitivity level. I just found out I have a severe B6 deficiency. I know many times people have both B6 and B12 deficiency.