What blood work is used to check vitamin B-12 levels?

Blood Work for Checking Vitamin B-12

Start with serum total B12 as the initial test, and if results are indeterminate (180-350 pg/mL), measure methylmalonic acid (MMA) to confirm functional deficiency. 1

First-Line Testing Approach

Serum Total B12 is the standard initial test for suspected vitamin B12 deficiency 1, 2:

• Cost-effective at approximately £2 per test with rapid turnaround time in local laboratories 1
• Diagnostic thresholds:
  • <180 pg/mL (<150 pmol/L): Confirms B12 deficiency—initiate treatment immediately 1, 2
  • 180-350 pg/mL (133-258 pmol/L): Indeterminate range—requires MMA testing 1, 2
  • >350 pg/mL: Makes deficiency unlikely 1

Alternative First-Line Test

Active B12 (Holotranscobalamin) measures the biologically active form available for cellular use 1, 3:

• More accurate than total B12 but costs £18 per test with longer turnaround times 1
• Diagnostic thresholds:
  • <25 pmol/L: Confirms deficiency 1
  • 25-70 pmol/L: Indeterminate—requires MMA testing 1
  • >70 pmol/L: Makes deficiency unlikely 1
• More sensitive than serum B12, detecting deficiency in patients with normal total B12 levels 3

Confirmatory Testing for Indeterminate Results

Methylmalonic Acid (MMA) is the gold standard for confirming functional B12 deficiency 1, 2:

• Measure when initial B12 results fall in the indeterminate range to identify functional deficiency that reflects actual cellular B12 status 1
• Detects an additional 5-10% of patients with B12 deficiency who have low-normal B12 levels 1
• 98.4% sensitivity for B12 deficiency, making it highly reliable 1
• More specific than homocysteine for B12 deficiency (MMA elevated in only 12.2% of folate-deficient patients versus 91% for homocysteine) 1
• Cost: £11-80 per test, requires specialized equipment 1

Important caveat: Both MMA and homocysteine can be falsely elevated in hypothyroidism, renal insufficiency, and hypovolemia—interpret cautiously in these conditions 1

Testing Algorithm for Patients Already Taking Supplements

If the patient is currently taking B12 supplements, measure MMA as the primary test 1:

• MMA reflects actual cellular B12 status regardless of supplementation 1
• Serum B12 testing has significant limitations and may not accurately reflect functional status in supplemented patients 1

If the patient is NOT taking supplements, start with total serum B12 or active B12 1

Additional Testing to Consider

Complete Blood Count (CBC) should be checked for megaloblastic anemia, though this may be absent in one-third of cases 1, 4

Homocysteine can be measured as an alternative functional marker, though it is less specific than MMA for B12 deficiency 1, 5, 6

Common Clinical Pitfalls

• Standard serum B12 testing misses functional deficiency in up to 50% of cases, as demonstrated in the Framingham Study where 12% had low serum B12 but an additional 50% had elevated MMA indicating metabolic deficiency 1
• Do not rely solely on serum B12 to rule out deficiency, especially in patients >60 years where metabolic deficiency is common despite normal serum levels 1
• Megaloblastic anemia can be masked by concurrent folic acid supplementation 4
• Functional markers (MMA, homocysteine) are more informative than serum B12 alone when assessing true deficiency status 1, 6