Martin is a 42-year-old male. He presented to his physician with complaints of mild memory loss. During his visit, Martin mentioned that he exercises at least 4 days a week and maintains a strict vegetarian diet.

Martin is seen by his physician who suspects a deficiency in his diet may be contributing to his neurological symptoms. His physician decides to order vitamin testing for more information.

the result of Martin’s serum vitamin B₁₂ laboratory test was 190 ng/L (Reference range 180–914 ng/L).

Although the result falls within the reference range, Martin’s physician doesn’t feel confident that he can rule out a deficiency.

The result of a serum methylmalonic acid (MMA) were 0.52 nmol/mL (reference range ≤ 0.40 nmol/mL). Given the moderately elevated MMA levels, Martin’s physician feels confident diagnosing him with vitamin B₁₂ deficiency.

What vitamin deficiency do you suspect in this patient? Why?

What are common methods for the determination of vitamin B₁₂?

What is another (potentially more definitive) test that may be ordered?

Why are these results more definitive than the vitamin B₁₂ results?