Vitamin B12 vs folate: two deficiencies that look alike on a CBC, and how to tell them apart

Both vitamins feed the same chemical step that lets dividing cells copy their DNA, so a shortage of either slows the marrow in the same way: red cells come out large (a high MCV), fewer and fragile, and neutrophils show extra lobes. From the CBC alone the two are indistinguishable, and the two deficiencies often coexist — in alcohol use, celiac disease, malnutrition and pregnancy. A high MCV, an unexplained anemia, or symptoms of either deficiency get both levels on the same draw, and treating one while the other is low leaves the anemia uncorrected.

The serum levels answer it in most people: a B12 under 200 pg/mL or a folate under about 3–4 ng/mL is low. Both have gray zones — B12 200–300 and folate 3–5 — where the metabolites decide: methylmalonic acid rises only when B12 is short, homocysteine rises in both. Red-cell folate reflects the last three months and is less affected by a recent meal than serum folate, which can normalize after a single folate-rich day.

History fills in the rest. Nerve symptoms, metformin, acid suppressants, gastric surgery or an autoimmune history point to B12; heavy drinking, pregnancy, methotrexate or a very poor diet point to folate. Low B12 has its own page.

Because the two vitamins share a pathway, a large dose of folic acid can push the blocked step forward even when B12 is missing. The anemia improves and the MCV falls — and the B12-related nerve damage carries on, sometimes to the point of permanent neuropathy or cognitive change. That is why guidelines insist on checking B12 before folic acid is started, why both are replaced when both are low, and why anyone on long-term folic acid with new numbness or unsteadiness needs a B12 level and a methylmalonic acid.

B12 deficiency is nearly always an absorption problem, so the fix is a high dose that bypasses it — 1,000–2,000 mcg a day by mouth works even in pernicious anemia, with injections for nerve symptoms or when tablets fail — and it usually continues for life. Folate deficiency is usually an intake or demand problem, so 1 mg of folic acid a day for one to four months refills the stores while the diet, drinking or medication is addressed; it stops when the cause is corrected, except in pregnancy planning, dialysis and chronic hemolysis, where a maintenance dose continues.