Vitamin B12 Test: Low, Normal, and the Band Where the Cutoffs Disagree

US labs report B12 in pg/mL and UK labs in ng/L, which are the same scale: 220 ng/L and 220 pg/mL are one result. Many European and Canadian labs use pmol/L, about 0.74 times the pg/mL figure, so 200 pg/mL is roughly 148 pmol/L.

The NIH Office of Dietary Supplements notes that most labs treat values below 200 or 250 pg/mL as low, and that the cutoff varies by method and laboratory. Labcorp's range starts at 232, so a result of 220 is flagged low there and printed as normal at a lab whose range starts at 200.

Low B12 affects the blood and the nerves, and the nerve symptoms often come first. On the blood side, fewer and larger red cells cause tiredness, breathlessness and pale skin; a raised MCV on a routine blood count is often how the problem is found. On the nerve side, people describe pins and needles or numbness starting in the hands and feet, unsteadiness in the dark, poor concentration and word-finding, low mood, and a sore, smooth, red tongue.

Nerve damage can become permanent if treatment is delayed once symptoms begin, so a borderline result with nerve symptoms is followed up promptly rather than simply repeated months later.

Folic acid can make this harder to see. In large amounts it corrects the anemia while the nerve damage continues, so the blood count looks better while the B12 deficiency goes on. That is why B12 is checked before a low folate is treated; the folate vs B12 comparison explains how the two are told apart.

These risks are also why some people are tested without symptoms, alone or as part of a vitamins and nutrients panel.

Serum B12 counts all the B12 in the blood, including the larger share bound to a carrier protein that cells cannot use. Only the smaller fraction bound to transcobalamin, called active B12 or holotranscobalamin, is delivered into cells. So a serum result can look adequate while cells are short, which is sometimes called functional B12 deficiency.

Three other tests help:

• Methylmalonic acid (MMA) builds up when cells lack B12 for one specific reaction. The NIH calls it the most sensitive marker of B12 status and treats a level above 0.271 µmol/L as suggesting deficiency; many US labs flag results above about 0.4. MMA also rises with reduced kidney function and tends to run higher in older adults. A normal MMA makes deficiency unlikely.
• Homocysteine rises above about 13 µmol/L when B12 is short, but also with low folate or B6, so it is sensitive but not specific.
• Active B12 measures the usable fraction directly. NICE treats it as an alternative first test, with its own bands (below 25 pmol/L deficient, 25 to 70 indeterminate, above 70 unlikely), and prefers it in pregnancy, when total B12 is unreliable.

A typical case for the second test: someone who has taken metformin for eight years, has numb toes and a B12 of 260 pg/mL. A raised MMA would confirm a deficiency that the serum result left open.

A B12 tablet the same morning, or an injection in recent weeks, raises the result. Someone already taking an over-the-counter B12 supplement can test normal with a deficiency still untreated. No fasting is needed, but Labcorp asks for the sample to be taken before B12 treatment starts and for high-dose biotin, common in hair and nail supplements, to be stopped at least 72 hours beforehand, because biotin interference can distort the B12 test.

A rarer error runs the other way. In pernicious anemia, high levels of intrinsic factor antibodies can disturb the test itself, and published case reports describe people with B12-deficiency anemia whose result read above 2,000 pmol/L.

A high B12 worries patients more than doctors. A daily supplement or a recent injection can keep the level above range for weeks, and B12 in multivitamins explains many of the rest. The Institute of Medicine set no upper limit for B12 intake because of its low potential for harm. A clearly raised level in someone taking no B12 at all is worth raising with a doctor, since liver disease and some bone marrow disorders can cause it.

Treatment depends on the cause and is the doctor's decision. About 1 percent of an oral dose is absorbed even without intrinsic factor, and the NIH summarizes trials in which 2,000 mcg a day, followed by a lower maintenance dose, may work as well as injections. Injections remain the standard when absorption has failed.

MMA and homocysteine fall within days to weeks of effective treatment, the blood count recovers over weeks, and nerve symptoms can take months and sometimes improve only partly. A serum B12 measured during treatment mostly reflects the treatment. Over the longer term, the direction matters more than a single value: a 320 means one thing if last year's result was 210 and another if it was 600, which is why following results over time suits B12 well.