Which B12 test actually tells you something

A serum B12 result of 240 pg/mL tells you less than it looks like it does. It is inside the "normal" range on most lab reports. It is also inside the range where a meaningful number of people turn out to be genuinely deficient.

The grey zone is not small

Laboratories usually flag deficiency below 200 pg/mL. Between 200 and 300 pg/mL — sometimes quoted up to 350 — the test cannot separate "low normal" from "deficient", because serum B12 measures what is circulating, not what is available to your cells.

Two things move that number without changing your actual B12 status: recent supplements (it goes up for weeks) and pregnancy (binding proteins rise, so total B12 looks higher). That is why the first rule is boring and important: test before you supplement.

Methylmalonic acid is the tie-breaker

When tissue B12 falls, methylmalonyl-CoA mutase slows and methylmalonic acid (MMA) accumulates. MMA is not influenced by recent supplementation the way serum B12 is, and it rises while serum B12 is still sitting in the grey zone.

In practice: serum B12 first, MMA when the result is borderline, and MMA again to confirm that treatment actually worked. A correction that normalises serum B12 but leaves MMA elevated usually means the dose is too low or absorption is the limiting problem.

Homocysteine: useful, but read it carefully

Homocysteine rises with low B12 and is the marker that links B12 status to vascular risk. It is sensitive but not specific — folate deficiency, low B6, kidney impairment, hypothyroidism and common genetic variants (MTHFR) all raise it.

So a high homocysteine does not prove B12 deficiency. It does say: something in this pathway needs a look, and B12 is the first candidate worth ruling out. The clinical value is that homocysteine is cheap, widely available, and responds within weeks to treatment — which makes it a decent follow-up marker even if it is a poor screening test.

A workable sequence

1. Serum B12 — with active symptoms, or with risk factors (plant-only diet, metformin, PPI, gut disease, age over 60).
2. MMA if serum B12 is 200–350 pg/mL, or if symptoms are neurological regardless of the number.
3. Homocysteine alongside, especially if vascular risk is the reason you are looking.
4. Full blood count with MCV — macrocytosis supports the diagnosis; a normal MCV does not exclude it.
5. If deficiency is confirmed, intrinsic factor and parietal cell antibodies to check for pernicious anaemia, which needs lifelong treatment rather than a course.

What not to do

Do not start a high-dose supplement and then test. You will get a normal serum B12 and an unresolved question. And do not treat a borderline result with reassurance alone if there are neurological symptoms — those can progress while the number sits comfortably in range.

Educational material, not medical advice.