Vitamin B12 200 to 300 pg/mL

This band is the one the bodies disagree about most, and the disagreement is about where it ends, not whether it exists. ODS calls 200 to 300 pg/mL "low or marginal vitamin B12 status," says it is "much more common" than deficiency with classic signs, "at up to 40% in Western populations," and advises checking methylmalonic acid (MMA) anywhere from 150 to 399 pg/mL. NICE's 2024 guideline sets its indeterminate zone at 180 to 350 ng/L (the same number as pg/mL) and labels it "possible vitamin B12 deficiency." The BSH guideline that NICE replaced put its indeterminate zone lower, at 150 to 200 ng/l. So a result of 320 is marginal to NICE and outside the ODS marginal band, though still inside the window where ODS advises an MMA check, and a result of 250 is in the zone for both. Ask which convention your report uses.

The second test. Every body agrees a marginal serum B12 is not a diagnosis. ODS: MMA "greater than 0.271 micromol/L suggests vitamin B12 deficiency," with the caveat that MMA also rises with kidney impairment and age; homocysteine over 15 micromol/L points the same way but is less specific. NICE recommendation 1.3.12: consider a serum MMA test in people with symptoms or signs and an indeterminate result. NICE also names active B12 (holotranscobalamin) as an acceptable first test, and it is the test NICE prefers in pregnancy, where total B12 falls without a true deficiency.

Symptoms decide what happens while you wait. NICE recommendation 1.3.14: consider starting replacement without waiting for the MMA result if you have an indeterminate result and a condition that may deteriorate rapidly (neurological or hematological, such as ataxia or anemia), a suspected irreversible cause such as autoimmune gastritis, surgery that causes deficiency, or you are pregnant or breastfeeding. With no symptoms or signs, NICE's answer is the opposite: no treatment, and "seek medical help if they develop symptoms or signs of deficiency." Its committee wrote that an incidental indeterminate result on routine blood work "would not need treatment unless they developed symptoms or signs."

What sits behind a marginal number. ODS lists the usual suspects: older age and atrophic gastritis, which affects 8 to 9 percent of adults 65 and older; metformin, which "might reduce the absorption of vitamin B12 and significantly reduce serum vitamin B12 concentrations"; proton pump inhibitors and H2 blockers, which slow the acid that frees B12 from food; and vegetarian or vegan diets. NICE adds recreational nitrous oxide, and notes the combined oral contraceptive pill lowers total B12 without causing deficiency. NICE also says people of Black ethnicity may have a higher reference range, so a result above a cutoff can still mean treatment is needed when symptoms fit. One statement worth carrying: NICE's committee acknowledged there is "no 'gold standard' test," which is why the number is read next to the symptoms and never alone.