Vitamin B12 Testing: When to Test, How to Interpret
B12 deficiency causes fatigue, tingling, and cognitive fog — and it is often missed because the standard reference range is too lax.
Who benefits from B12 testing
Adults over 50 (absorption naturally declines), long-term users of metformin or proton-pump inhibitors, people on strict plant-based diets, patients with pernicious anemia or gastric surgery, and anyone with unexplained fatigue, neuropathy, or cognitive symptoms.
Interpretation and confirmatory tests
Serum B12 above 220 pmol/L is generally sufficient; below 150 pmol/L confirms deficiency. The grey zone (150–220) accounts for a large share of missed cases. In that zone, methylmalonic acid (elevated in true deficiency) or homocysteine confirms tissue-level status.
Treatment is safe even when in doubt: oral cyanocobalamin 1000 µg daily, or intramuscular hydroxocobalamin injections for absorption problems. See B12 injections.
Test B12 as part of your bloodwork
Ready to book? Reserve a private blood collection appointment on our Book Blood Collection page, or request a private requisition through blood test prescription.
Frequently asked questions
Do I need to fast for a B12 test?+
No — fasting is not required.
Can supplements distort the result?+
Recent high-dose B12 supplements can elevate the serum value; hold for 24–48 hours before testing if your clinician suspects true deficiency.