Vitamin B12 Testing: Levels, Deficiency & When to Check
Content last reviewed 2026-09-12 · Educational information — not medical advice
Quick answer
B12 testing catches the deficiency behind anemia, nerve tingling, and cognitive fog — often years before symptoms peak because your liver stores last so long. Serum levels 200–350 pg/mL live in a gray zone where methylmalonic acid testing reveals what B12 alone misses.
What does a vitamin B12 test measure?
Circulating cobalamin attached to transport proteins. Your body warehouses YEARS of supply in the liver, so serum levels decline slowly — deficiency develops insidiously through absorption failures (pernicious anemia's intrinsic-factor loss, gastric surgery, Crohn's terminal-ileum disease) rather than dietary gaps alone. Long-term metformin and PPI use drain levels pharmaceutically; strict veganism without supplementation depletes stores over years.
Symptoms worth testing for
Blood side: macrocytic anemia (big pale cells flagged on CBC), fatigue, pallor, glossitis. Nerve side: tingling/numbness climbing feet-to-legs, balance trouble, memory fog, mood changes. The neurological damage can become permanent if correction delays — and can appear WITHOUT any anemia whatsoever.
Reading your number honestly
Below 200 pg/mL = deficient, treat. Above 350 = fine. The 200–350 gray zone is where plain serum B12 fails roughly a quarter of deficient people — functional markers settle it: methylmalonic acid (rises specifically in B12 deficit; folate problems don't move it) ± homocysteine (rises in either deficiency).
Oral supplements versus shots
High-dose oral B12 (1000–2000µg daily) matches injections for most causes via passive gut diffusion — 1% absorbs without intrinsic factor, which suffices at those doses. Injections remain preferred for severe neurological presentations, malabsorption confirmed cases, and when adherence doubts exist.
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Frequently asked questions
How long until I feel better after starting B12?
Energy often improves within days-to-weeks; nerve symptoms recover over months and may plateau permanently if damage was longstanding.
Do energy drinks with B12 help fatigue?
Only if you're actually deficient. In replete people, extra B12 is expensive urine — the marketing inverts the evidence.