Vitamin B12 under 200 pg/mL

The NIH Office of Dietary Supplements puts the line where most laboratories put it: "most laboratories define subnormal serum or plasma values as those lower than 200 or 250 picograms per milliliter." Two conventions, not a range, and yours depends on the lab. Three bodies draw the deficiency line in different places and different units: ODS at 200 or 250 pg/mL, the British Society for Haematology (BSH) at 148 pmol/l (200 ng/l, the same number as 200 pg/mL), and NICE's 2024 guideline at under 180 ng/L, which it labels "Confirmed vitamin B12 deficiency." Below 180 pg/mL you are under all three. Between 180 and 200 you are under the ODS and BSH lines but inside NICE's indeterminate band.

What it means. The effects ODS lists run from the hallmark megaloblastic anemia and low blood counts through glossitis, fatigue, palpitations, dementia and weight loss to numbness and tingling in the hands and feet, and it warns that "These neurological symptoms can occur without anemia, so early diagnosis and intervention is important to avoid irreversible damage." NICE says the same in its own way: "Do not rule out a diagnosis of vitamin B12 deficiency based solely on the absence of either anaemia or macrocytosis." Because the body stores 1 to 5 mg, ODS notes symptoms can take years to appear, which is why a low number can be new to you and old to your body.

What comes next. NICE's rule is to draw diagnostic blood before starting replacement, and to ask whether you already take an over-the-counter B12 product, because that can raise the number "without fully treating a deficiency." The cause decides the route. NICE offers lifelong intramuscular replacement when autoimmune gastritis is the cause or suspected cause, or after total gastrectomy or complete terminal ileal resection, and says to consider injections over tablets for other malabsorption (celiac disease, partial gastrectomy, some bariatric surgery). Where tablets are used for malabsorption, the dose is "at least 1 mg a day." For a deficiency caused by a medicine, either route is acceptable while the medicine continues, with a review of whether the medicine is still needed. For a dietary cause, oral replacement and a supplement containing cyanocobalamin, methylcobalamin or adenosylcobalamin. ODS reports that a 2018 Cochrane review found high oral doses (1,000 to 2,000 mcg) normalized serum B12 about as well as injections, on low-quality evidence.

Do not wait if nerves are involved. NICE recommendation 1.3.6: do not delay replacement while waiting for results in people with suspected megaloblastic anemia and neurological symptoms. The 2014 BSH guideline, now superseded by NICE but consistent with it, said "treatment should not be delayed to avoid neurological impairment." NICE's first follow-up is at 3 months, or 1 month in pregnancy or breastfeeding, and it says symptoms may start improving within 2 weeks but can take up to 3 months. If you are on injections, NICE says not to repeat the initial test; the number is no longer the question.