What is the difference between vitamin B12 and active B12 assay in diagnosing vitamin B12 deficiency?

Vitamin B12 vs Active B12 Assay

Active B12 (holotranscobalamin) is a more sensitive and specific marker than total serum B12 for diagnosing vitamin B12 deficiency, as it measures the biologically active form available for cellular use, and should be measured when total B12 results are indeterminate or when clinical suspicion remains high despite normal total B12 levels. 1

The Critical Limitation of Standard B12 Testing

• Up to 50% of patients with "normal" total serum B12 levels actually have functional B12 deficiency when measured by metabolic markers like methylmalonic acid (MMA). 2, 3
• Standard total B12 tests measure both the active and inactive forms of vitamin B12, which may not accurately reflect the biologically active form available for cellular use. 1
• This explains why patients can have neurological symptoms and metabolic deficiency despite "normal" serum B12 levels. 2

What Active B12 (Holotranscobalamin) Measures

• Active B12 specifically measures holotranscobalamin, which is the biologically active form of B12 bound to transcobalamin II that is actually available for cells to use. 1, 4
• This marker is more sensitive than total B12 and is increasingly being introduced in clinical settings as a superior diagnostic test. 4
• The British Medical Journal recommends measuring active B12 (holotranscobalamin) if only total B12 was initially tested. 1

Diagnostic Algorithm for B12 Deficiency

Initial Testing:

• Start with total serum B12 level as the primary screening test. 5
• Levels <180 pg/mL (<150 pmol/L) are diagnostic for deficiency. 1, 5

Borderline or Indeterminate Results (180-350 pg/mL):

• Measure methylmalonic acid (MMA), which is diagnostic for B12 deficiency if elevated. 1, 5
• Consider measuring active B12 (holotranscobalamin) as it is more sensitive than total B12. 1, 4
• Elevated MMA confirms functional B12 deficiency even when serum B12 appears normal. 1

When Clinical Suspicion Remains High Despite Normal Total B12:

• Measure active B12 (holotranscobalamin) to detect functional deficiency. 1
• MMA testing should be performed as it is a more sensitive marker of functional B12 status. 1
• Homocysteine levels can also suggest functional deficiency. 1

Clinical Pitfalls to Avoid

• Never rely solely on total serum B12 levels to exclude deficiency, as this misses up to 50% of patients with metabolic deficiency. 2, 1
• Neurological symptoms often appear before hematological changes and can become irreversible if untreated. 1, 6
• One-third of B12 deficiency cases may not show megaloblastic anemia on complete blood count. 1
• Normal reference ranges vary between laboratories and may not be appropriate for all individuals. 1

High-Risk Populations Requiring Lower Threshold for Testing

• Age ≥60 years (25% of those ≥85 years have deficiency). 1
• Patients on metformin (especially >4 months), proton pump inhibitors, H2 receptor antagonists, colchicine, phenobarbital, pregabalin, or primidone. 2, 1, 5
• Vegan/vegetarian diets or restricted dietary intake. 2, 1
• Atrophic gastritis, celiac disease, or autoimmune conditions (thyroid disease, type 1 diabetes, Sjögren syndrome). 2, 1
• Post-bariatric surgery patients. 1