What is the recommended workup and treatment for a vitamin B12 deficiency?

Vitamin B12 Deficiency Workup

Initial Testing Strategy

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

First-Line Test Options

• Total B12 (serum cobalamin): Costs approximately £2 per test with rapid turnaround in local laboratories 2
• Active B12 (holotranscobalamin): Measures biologically active B12 available for cells, costs £18 per test with longer turnaround times 2

Both are acceptable initial tests according to NICE 2024 guidelines, with the choice depending on local laboratory availability and cost considerations 1.

Interpreting Initial B12 Results

For Total B12:

• <180 ng/L (133 pmol/L): Confirmed deficiency—start treatment immediately 1
• 180-350 ng/L (133-258 pmol/L): Indeterminate—proceed to MMA testing 1
• >350 ng/L (258 pmol/L): Deficiency unlikely 1

For Active B12:

• <25 pmol/L: Confirmed deficiency—start treatment immediately 1
• 25-70 pmol/L: Indeterminate—proceed to MMA testing 1
• >70 pmol/L: Deficiency unlikely 1

Second-Line Testing: Methylmalonic Acid

Order MMA testing when initial B12 results are indeterminate (borderline) to identify functional B12 deficiency. 1, 2

Why MMA Matters

MMA is critical because standard serum B12 testing misses functional deficiency in up to 50% of cases—in the Framingham Study, 12% had low serum B12, but an additional 50% had elevated MMA indicating metabolic deficiency despite "normal" B12 levels 2. MMA has 98.4% sensitivity for B12 deficiency and is more specific than homocysteine 2.

MMA Interpretation

• Elevated MMA (>271 nmol/L or >0.26 μmol/L): Confirms functional B12 deficiency even with normal serum B12 2
• MMA detects an additional 5-10% of patients with B12 deficiency who have low-normal B12 levels 2
• In polyneuropathy patients, 44% had B12 deficiency based solely on abnormal metabolites when serum B12 was normal 2

Cost-Effectiveness Considerations

MMA testing costs £11-80 per test and requires specialized equipment 1. However, cost-effectiveness analysis shows MMA testing before treatment is cost-effective at £3,946 per quality-adjusted life year when B12 results are indeterminate 2. Using MMA as first-line screening is not cost-effective due to expense and processing delays 2.

Critical caveat: Both MMA and homocysteine can be falsely elevated in hypothyroidism, renal insufficiency, and hypovolemia—interpret cautiously in these conditions 2.

Complete Workup Components

Minimum Initial Laboratory Assessment

• Complete blood count (CBC) with red cell indices (MCV, RDW) 1, 3
• Serum B12 (total or active) 1, 3
• Reticulocyte count 1
• Serum ferritin and transferrin saturation 1
• C-reactive protein (CRP) 1

Extended Workup When Cause Remains Unclear

• Methylmalonic acid (if B12 indeterminate) 1, 2
• Homocysteine (target <10 μmol/L for optimal outcomes) 2
• Vitamin B12 and folic acid levels 1
• Percentage of hypochromic red cells 1
• Reticulocyte hemoglobin 1
• Lactate dehydrogenase 1
• Haptoglobin 1

Clinical Assessment: Who to Test

High-Risk Populations Requiring Screening

Screen patients with any of these risk factors: 4, 3, 5

• Dietary: Vegans, strict vegetarians, limited fortified food consumption 4
• Gastrointestinal: Gastric/small intestine resections, inflammatory bowel disease (especially ileal Crohn's >30-60 cm involvement), celiac disease, atrophic gastritis 4, 3
• Medications: Metformin >4 months, proton pump inhibitors or H2 blockers >12 months, colchicine, phenobarbital, pregabalin, primidone 4, 3
• Age: Adults >75 years (25% of those ≥85 years have B12 <170 pmol/L) 2
• Autoimmune conditions: Thyroid disease, Sjögren syndrome, type 1 diabetes 4
• Post-bariatric surgery: All types, especially Roux-en-Y gastric bypass 2, 4

Specific Symptoms and Signs to Assess

Hematological manifestations: 4, 5

• Fatigue (most common, present in 66.7% of patients) 6
• Macrocytic anemia (MCV >100 fL) 6
• Glossitis (tongue inflammation) 4

Neurological manifestations (often appear before hematological changes): 4, 5

• Peripheral neuropathy: tingling, numbness, paresthesias (54.4% of patients) 4, 6
• Balance issues and falls from impaired proprioception/sensory ataxia 4
• Cognitive difficulties, memory problems, brain fog 2, 5
• Blurred vision from optic nerve dysfunction 4

Critical warning: Neurological symptoms often present before hematological changes and can become irreversible if untreated 2. Vitamin B12 deficiency that progresses for longer than 3 months may produce permanent degenerative lesions of the spinal cord 7.

Identifying the Underlying Cause

Tests to Determine Etiology

Once deficiency is confirmed, determine the cause to guide treatment duration:

For suspected pernicious anemia/autoimmune gastritis: 2

• Intrinsic factor antibodies (if positive, lifelong treatment required) 2
• Gastrin levels (markedly elevated >1000 pg/mL indicates pernicious anemia) 2
• Helicobacter pylori testing 5
• Evaluation for autoantibodies associated with autoimmune gastritis 5

For suspected malabsorption: 1, 4

• Review surgical history (ileal resection, bariatric surgery) 4
• Assess for inflammatory bowel disease 1
• Screen for celiac disease (tissue transglutaminase antibodies and total IgA) 2

Common Pitfalls to Avoid

1. Never rely solely on serum B12 to rule out deficiency, especially in patients >60 years where metabolic deficiency is common despite normal serum levels 2

2. Never administer folic acid before treating B12 deficiency—folic acid can mask anemia while allowing irreversible neurological damage to progress (subacute combined degeneration of the spinal cord) 4, 8, 7

3. Don't assume normal B12 excludes deficiency in high-risk populations—18.1% of patients >80 years have metabolic B12 deficiency despite "normal" serum B12 2

4. Don't stop at one normal result—patients with malabsorption or dietary insufficiency often require ongoing supplementation and can relapse 8

5. Don't ignore borderline results in symptomatic patients—proceed to MMA testing to confirm functional deficiency 1, 2

Special Population Considerations

Post-Bariatric Surgery Patients

Screen at 3-month intervals if planning pregnancy, otherwise annually 8. These patients have permanent malabsorption and require lifelong supplementation 8. Deficiency can occur even when serum concentrations are 300 pmol/L 2.

Autoimmune Thyroid Disease

Screen all patients with autoimmune hypothyroidism at diagnosis and annually thereafter—prevalence of B12 deficiency ranges from 28-68% and is strongly associated with positive thyroid antibodies 2. Also test for celiac disease, as 13.3% of B12-deficient hypothyroid patients have positive tissue transglutaminase antibodies 2.

Elderly Patients (>60 years)

This population has significantly higher rates of metabolic deficiency despite "normal" serum B12—18.1% of patients >80 years have metabolic B12 deficiency 2. Consider MMA testing even with low-normal B12 levels in symptomatic elderly patients 2.