Vitamin B12 (cobalamin) deficiency may be difficult to identify from serum cobalamin levels alone. In patients biochemically deficient for cobalamin, methylmalonic acid (MMA) is increased in the serum and urine, but normalizes after treatment with vitamin B12. MMA testing can be expensive and may not be readily available but can help identify patients with vitamin B12 deficiency before irreversible neurological damage has occurred.

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