What is the initial workup for a patient with suspected vitamin B12 deficiency?

Workup for Vitamin B12 Deficiency

Start with either total serum B12 or active B12 (holotranscobalamin) as your initial test, then add methylmalonic acid (MMA) testing only when results fall in the indeterminate range to confirm functional deficiency. 1

Initial Laboratory Testing

Choose one of these first-line tests:

• Total serum B12 (serum cobalamin): The most cost-effective initial test at £2 per test with rapid turnaround in local laboratories 1
• Active B12 (serum holotranscobalamin): More accurate as it measures the biologically active form available for cellular use, but costs £18 per test with longer turnaround times due to external laboratory processing 1

Both tests are acceptable initial choices - the guideline explicitly states that despite active B12 being more accurate, insufficient evidence exists to mandate this more expensive test as standard practice 1

Interpreting Initial Test Results

If Testing Total B12:

• <180 ng/L (133 pmol/L): Confirmed B12 deficiency - proceed to treatment 1
• 180-350 ng/L (133-258 pmol/L): Indeterminate result - measure MMA to confirm functional deficiency 1
• >350 ng/L (258 pmol/L): B12 deficiency unlikely 1

If Testing Active B12:

• <25 pmol/L: Confirmed B12 deficiency - proceed to treatment 1
• 25-70 pmol/L: Indeterminate result - measure MMA to confirm functional deficiency 1
• >70 pmol/L: B12 deficiency unlikely 1

Confirmatory Testing with Methylmalonic Acid

When to order MMA:

• Only when initial B12 results fall in the indeterminate range AND clinical suspicion remains high based on symptoms or risk factors 1

Why MMA is superior for confirmation:

• Detects an additional 5-10% of patients with functional B12 deficiency who have low-normal B12 levels 2
• Has 98.4% sensitivity for B12 deficiency 2
• More specific than homocysteine for B12 deficiency (MMA elevated in only 12.2% of folate-deficient patients versus 91% for homocysteine) 2
• In polyneuropathy patients, 44% had B12 deficiency based on abnormal MMA when serum B12 was normal 2

Cost-effectiveness: MMA testing before treatment is cost-effective at £3,946 per quality-adjusted life year when B12 results are indeterminate, but is not appropriate as first-line screening due to expense (£11-80 per test) and processing delays 1, 2

Additional Workup to Identify Underlying Cause

Complete blood count with differential:

• Check for macrocytic anemia (though megaloblastic anemia is absent in one-third of cases) 2, 3
• Note that concurrent folic acid supplementation can mask megaloblastic anemia 3

For patients without clear dietary or medication-related causes:

• Test for Helicobacter pylori to evaluate for atrophic gastritis 4
• Measure intrinsic factor antibodies and parietal cell antibodies to diagnose autoimmune gastritis/pernicious anemia 4, 5
• Consider testing for celiac disease (tissue transglutaminase antibodies with total IgA), especially in patients with autoimmune thyroid disease where 13.3% have positive celiac markers 2

In patients with autoimmune hypothyroidism:

• Screen all patients at diagnosis and annually thereafter, as B12 deficiency prevalence ranges from 28-68% in this population 2

Critical Clinical Pitfalls to Avoid

Standard serum B12 testing misses up to 50% of functional deficiencies - the Framingham Study showed that while 12% had low serum B12, an additional 50% had elevated MMA indicating metabolic deficiency despite "normal" B12 levels 2, 6

False elevations of MMA and homocysteine occur in:

• Hypothyroidism 2
• Renal insufficiency 2
• Hypovolemia 2

Do not rely solely on serum B12 to rule out deficiency in patients >60 years, where metabolic deficiency is common (18.1% overall, 25% in those ≥85 years) despite normal serum levels 2

Medications that interfere with B12 absorption or utilization:

• Metformin (especially if used >4 months) 2, 7, 4
• Proton pump inhibitors or H2 receptor antagonists (especially if used >12 months) 2, 7
• Colchicine, phenobarbital, pregabalin, primidone 2, 3

Special Populations Requiring Different Approach

Pregnant patients and those with nitrous oxide-related deficiency: The standard testing algorithm does not apply - refer to full NICE guideline for specific guidance 1

Post-bariatric surgery patients: High risk for malabsorption due to reduced gastric acid and intrinsic factor; deficiency may present gradually as B12 stores last 2-3 years 2