Should I check the patient's vitamin B12 (B12) level?

Vitamin B12 Testing Recommendations

Yes, you should check the patient's vitamin B12 level as part of a comprehensive metabolic assessment, especially if there are risk factors for deficiency or symptoms that could be attributed to low B12. 1

Indications for B12 Testing

B12 testing is warranted in the following situations:

• Neurological symptoms (peripheral neuropathy, ataxia, cognitive impairment)
• Hematologic abnormalities (macrocytic anemia, elevated MCV)
• Elevated homocysteine levels
• High-risk populations:
  • Elderly patients (>75 years)
  • Patients with malabsorption disorders
  • Post-bariatric surgery patients
  • Patients with Crohn's disease (especially with ileal involvement)
  • Vegans and vegetarians
  • Patients on long-term metformin or proton pump inhibitors 1, 2, 3

Proper Testing Approach

When checking B12 status, follow this approach:

1. Initial testing: Order serum total B12 (cobalamin) or active B12 (holotranscobalamin)

   • Interpret results according to these thresholds:
     • <180 ng/L (total B12) or <25 pmol/L (active B12): Confirmed deficiency
     • 180-350 ng/L (total B12) or 25-70 pmol/L (active B12): Indeterminate

     • 350 ng/L (total B12) or >70 pmol/L (active B12): Unlikely deficiency 1

2. For indeterminate results: Order additional testing

   • Methylmalonic acid (MMA) - more specific marker for B12 deficiency
   • Homocysteine levels - elevated in both B12 and folate deficiency
   • Complete blood count - to assess for macrocytic anemia
   • Folate levels - often coexists with B12 deficiency 1, 4

Important Clinical Considerations

• Normal serum B12 levels do not rule out deficiency. Up to 18.1% of elderly patients have metabolic B12 deficiency despite normal serum B12 levels 1, 3

• Vitamin B12 deficiency that progresses for longer than 3 months may produce permanent degenerative lesions of the spinal cord 5

• Early detection and treatment of B12 deficiency can prevent serious and permanent hematologic and neurological damage 2

• Patients with pernicious anemia have about 3 times the incidence of carcinoma of the stomach as the general population, warranting appropriate testing when indicated 5

Potential Pitfalls in B12 Testing

• Certain medications can affect B12 test results:

  • Most antibiotics
  • Methotrexate
  • Pyrimethamine
  • Colchicine
  • Para-aminosalicylic acid
  • Heavy alcohol intake (>2 weeks) 5, 4

• Folic acid supplementation can mask B12 deficiency hematologically while allowing neurological damage to progress 5

• Relying solely on serum B12 without considering clinical presentation may miss true deficiency, especially in patients with neurological symptoms 3, 4

By checking vitamin B12 levels and following up with appropriate additional testing when indicated, you can identify and treat deficiency before it leads to irreversible neurological damage or other serious complications.