Answer:

Check a methylmalonic acid level

This patient has had a history of gastric cancer with a partial gastrectomy and now presents with a fatigue and gait disturbances.  CBC reveals a macrocytic aneamia and peripheral smear shows multinucleated neutrophils.  The gait disturbances are most likely due to the progressive peripheral neuropathy.  This is most consistent with vitamin B12 deficiency.  Intrinsic factor, which is secreted by the parietal cells of the stomach, is required for vitamin B12 absorption in the terminal ileum.

Work-Up for Vitamin B12 Deficiency

Serum B12 Level

• < 300 pg/dL is diagnostic

Metobolites

• Methylmalonic acid
  • < 70 nanomol/L is diagnostic
• Homocysteine
  • < 5 micromol/L is diagnostic

Possible additional testing in the setting of macrocytic anemia:

• If pernicious anemia is suspected:
  • Anti-intrinsic factor antibodies
• If folate deficiency is suspected:
  • Serum folate level
    • < 2 ng/mL is diagnostic
  • RBC folate level (reserved for indeterminate serum levels)
    • < 280 ng/mL is diagnostic

Treatment for B12 Deficiency

• Intramuscular
  • 1mg daily x 7 days, then 1mg weekly for 4 weeks, then 1mg monthly
• Oral
  • 1000-2000mg daily

References

1. Antony AC. Megaloblastic anemias. In: Hematology: Basic principles and practice, 4th ed, Hoffman R, Benz EJ, Shattil SJ, et al. (Eds), Churchill Livingstone, New York 2005. p.519.
2. Butler CC, Vidal-alaball J, Cannings-john R, et al. Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency: a systematic review of randomized controlled trials. Fam Pract. 2006;23(3):279-85.