Test your medicine knowledge with the MKSAP challenge, in partnership with the American College of Physicians.

A 57-year-old woman is evaluated for a 2-week history of decreased exercise tolerance and substernal chest pain on exertion. She also has an 8-month history of macrocytic anemia.

On physical examination, temperature is 36.7 °C (98.0 °F), blood pressure is 137/78 mm Hg, pulse rate is 104/min, and respiration rate is 17/min. BMI is 25. The patient has pale conjunctivae. Cardiopulmonary and neurologic examination findings are normal.

Initial laboratory studies indicate a hemoglobin level of 7.4 g/dL (74 g/L), a mean corpuscular volume of 104 fL, a serum vitamin B12 level in the low-normal range, and a normal red cell folate level. Subsequent testing indicates elevated serum homocysteine and methylmalonic acid levels.

An electrocardiogram is normal.

Which of the following is the most likely diagnosis?

A: Cobalamin deficiency
B: Combined folate and cobalamin deficiency
C: Folate deficiency
D: Transcobalamin II deficiency

MKSAP Answer and Critique

The correct answer is A: Cobalamin deficiency.

The most likely diagnosis is cobalamin (vitamin B₁₂) deficiency. Patients with vitamin B₁₂ deficiency have elevated homocysteine and methylmalonic acid levels, whereas patients with folate deficiency have only an elevated homocysteine level. In addition, an elevated methylmalonic acid level is more sensitive and specific for diagnosing vitamin B₁₂ deficiency than a low serum vitamin B₁₂ level because serum vitamin B₁₂ levels do not adequately assess tissue vitamin B₁₂ stores, especially in patients with vitamin B₁₂ levels in the low-normal range. Consequently, homocysteine and methylmalonic acid should be measured in patients with suspected vitamin B₁₂ deficiency. Similarly, red blood cell folate can be low in patients with folate or vitamin B₁₂ deficiency. Because folate supplementation can correct the anemia of vitamin B₁₂ deficiency but not the progression of neurologic defects, vitamin B₁₂ deficiency must be excluded before supplemental folate is administered to a patient with macrocytic anemia and a low red cell folate level.

Patients with vitamin B₁₂ deficiency have elevated homocysteine and methylmalonic acid levels, whereas patients with folate deficiency have only an elevated homocysteine level. Therefore, this patient does not have folate or combined folate-cobalamin deficiency.

Patients with transcobalamin II deficiency have normal serum vitamin B₁₂ levels because transcobalamin II is the primary transporter protein for vitamin B₁₂ entry into cells. Deficiency of transcobalamin II is quite rare and typically presents in childhood as a megaloblastic anemia with normal vitamin B₁₂ and red cell folate levels.

Key Point

• An elevated serum methylmalonic acid level is more sensitive and specific for diagnosing cobalamin (vitamin B₁₂) deficiency than a low serum vitamin B₁₂ level.

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