Evidence shows that chronic use of metformin (longer than one year) causes vitamin B12 deficiency. I routinely screen all my diabetic patients for a B12 level, especially those with complaints of neuropathy.

What is the treatment goal for B12 in a patient with diabetic neuropathy, given the wide range of normal (200-950 pg/mL)? If a patient’s B12 is on the lower end of normal and the methylmalonic acid (MMA) level is normal, would you treat him or her? Would you routinely draw a methylmalonic acid level with every B12 level? — Kristin Andrs, NP, CCRN, CDE, Richmond, Va.

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In approximately 30% of diabetics, metformin blocks absorption of vitamin B12 by inhibiting calcium-channel pathways in the intestines. It is most appropriate to treat these patients with both calcium and vitamin B12 supplementation. Unfortunately, there is no research or guideline to tell us what B12 levels are optimal.

It is known that MMA levels begin to rise when B12 is <400 pg/mL and that neurologic impairment begins at the same level. It seems reasonable to get patients above that. With regard to simultaneously testing MMA and B12, it depends on the insurance companies — will they cover MMA before it is known if the patient is B12 deficient?

I have had a number of symptomatic patients with B12 levels <400 pg/mL who have normal MMA measurements. I treat B12 deficiency regardless of MMA level, especially since there is a limited window of opportunity to reverse neurologic impairment. — Rebecca H. Bryan, APRN, CNP (154-12)