Vitamin B12 Deficiency Blood Test: Ranges, Symptoms and What to Do

Why B12 Matters

Vitamin B12 is essential for making red blood cells, maintaining the myelin sheath that insulates nerve fibres, producing DNA, and supporting brain function. Unlike most water-soluble vitamins, B12 can be stored in the liver — but those stores can take years to deplete, which is why deficiency develops slowly and is so often caught late.

B12 is found almost exclusively in animal products — meat, fish, dairy, and eggs. People who follow vegan or vegetarian diets for extended periods without supplementing will almost certainly become deficient. B12 is also poorly absorbed in the presence of low stomach acid, which becomes more common with age and with the use of proton pump inhibitors (PPIs) and metformin.

B12 Blood Test Ranges — The Problem with "Normal"

Category	Serum B12 Level
Deficient	Below 200 pg/mL
Borderline / grey zone	200–300 pg/mL
Lab "normal" (lower bound)	200–250 pg/mL (varies by lab)
Functional optimum	400–900 pg/mL
High (usually benign)	Above 900 pg/mL

The critical issue with the standard serum B12 test is the grey zone between 200–300 pg/mL. Many US labs report the lower end of "normal" at 200–250 pg/mL — meaning you could be told your B12 is fine at 210 pg/mL while experiencing neurological symptoms of deficiency. Studies show that functional B12 deficiency can occur at levels up to 350–400 pg/mL in some people, particularly older adults.

If your B12 is in the 200–350 pg/mL range and you have symptoms, ask your doctor to test methylmalonic acid (MMA) and homocysteine — both are more sensitive markers of functional B12 deficiency. Elevated MMA is highly specific for B12 deficiency even when serum B12 appears borderline.

Symptoms of B12 Deficiency

B12 deficiency affects two main systems — the nervous system and the blood — and symptoms from each develop at different rates:

Neurological symptoms (often earliest): tingling or numbness in hands and feet, difficulty walking or balance problems, memory problems and cognitive decline, depression, irritability, brain fog, and in severe deficiency, nerve damage that can become permanent if untreated.

Blood symptoms (megaloblastic anemia): fatigue and weakness, pale or yellow skin, shortness of breath, a sore or inflamed tongue (glossitis). On your CBC, B12 deficiency shows as a high MCV (large red blood cells) with low hemoglobin.

The neurological symptoms of B12 deficiency can occur even without anemia — in fact, treating the anemia with folate can mask B12 deficiency while the neurological damage continues, which is why B12 and folate must always be tested together.

Who Is at Highest Risk of B12 Deficiency?

Vegans and strict vegetarians, people over 50 (stomach acid declines with age, reducing B12 absorption from food), people taking metformin (widely used for pre-diabetes and type 2 diabetes — metformin reduces B12 absorption by up to 30%), people on long-term PPIs (omeprazole, pantoprazole), people who have had gastric bypass surgery, and anyone with autoimmune gastritis or pernicious anemia (which destroys the intrinsic factor needed to absorb B12).

Treatment: Supplements vs Injections

For dietary deficiency: Healthcare providers commonly recommend high-dose oral B12 supplementation (commonly 1,000 mcg daily — discuss the specific form and dose with your healthcare provider). At sufficiently high doses, a small percentage of B12 is absorbed passively without requiring intrinsic factor — which makes oral supplementation effective even in some absorption disorders. Confirm the appropriate form and dose with your healthcare provider.

For severe deficiency or absorption disorders: Intramuscular (IM) B12 injections are a prescription treatment that bypasses the gut entirely. They are typically used for pernicious anemia, severe neurological symptoms, or confirmed absorption failure. Your doctor will determine the appropriate protocol based on your individual situation and the underlying cause of deficiency.

Form of B12: Methylcobalamin and adenosylcobalamin are the active forms found in food. Cyanocobalamin (the synthetic form in most supplements) must be converted by the body. For most healthy people either works, but methylcobalamin is preferred for neurological conditions.

Monitoring: After starting supplementation, retest B12 in 3 months to confirm levels are rising. If MMA was elevated, retest that too — it's more sensitive for confirming adequate treatment response than serum B12 alone.