06/21/2026
This is why it's so important to look beyond blood work as a lot can happen before it shows up in the blood. A multitude of testing, symptom picture analysis plus blood work is best.
In a 1988 New England Journal study of 141 consecutive patients with neurological and psychiatric symptoms from B12 deficiency, 28 percent (40 patients) lacked the classic macrocytic anemia, the combined low hemoglobin plus enlarged red cells that flags B12 deficiency on the standard CBC. Within that group, 19 had completely normal CBCs and 18 had borderline or normal serum B12 despite clear neurological deficiency.
Your liver stores enough B12 to last several years. As intake drops or absorption fails, the body draws on those stores. The active form of B12 in blood (holotranscobalamin) falls first because it is the metabolically delivered fraction. Total serum B12 falls later. Methylmalonic acid (MMA) and homocysteine rise as soon as B12 supply at the tissue level becomes inadequate, because the two enzymes that need B12 begin to stall. The CBC moves last because red cells live about 120 days, and changes in their average size take months to register.
B12 is needed to make methionine, and methionine is the methyl donor your nerves use to maintain the myelin sheath that insulates them. When methyl-donation falters, myelin maintenance falters. Peripheral nerves register tingling, numbness, and balance problems. The brain registers memory and mood changes. Lindenbaum's series included tingling, numbness, balance problems, dementia, and psychiatric disorders in patients without the classic macrocytic anemia signature.
So who is likely to get this? Strict vegetarians and vegans (B12 comes from animal sources or fortified foods). Older adults whose gastric acid drops with age (acid is needed to free B12 from food protein). People on long-term acid-suppressing medication for the same reason. People on metformin, which impairs B12 absorption. People who have had gastric bypass or part of the small intestine removed. Pernicious anemia (autoimmune attack on the gastric cells that produce intrinsic factor, the protein B12 needs to be absorbed in the gut).
Serum B12 alone is a weak test. MMA and holotranscobalamin catch functional deficiency much earlier. The CBC catches it last, and by the time the CBC abnormality appears, the nerve damage may already be substantial. Lindenbaum found that treatment improved neuropsychiatric symptoms in 39 of 39 patients evaluated after therapy. That is the good news. The hard part is catching it before the damage outpaces recovery.
Lindenbaum NEJM 1988
Stabler NEJM 2013
Green Blood 2017
