Does normal methylmalonic acid levels absolutely rule out vitamin B12 deficiency?  

An elevated MMA is a good indicator of B-12 deficiency but a normal level does not argue strongly against B-12 deficiency. Normal MMA levels have been reported in 10-25% of patients with known B-12 deficiency (B-12 levels below 100). Testing for an elevated homocysteine level might help but this tes...