Do 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 test is usually a better indicator of folate deficiency than B-12. If the B-12 level is clearly low or even borderline, a trial of B-12 could be employed while you search for other causes of the symptoms that led you to do the MMA.