What serum B12 actually measures, why methylmalonic acid is the more sensitive marker, and the metformin problem hiding inside diabetic neuropathy
“Your B12 was normal” is one of the most common sentences in a neuropathy workup, and one of the least conclusive. The test it refers to is less reliable than its routine use suggests, and there is a better one that is frequently never ordered.
What serum B12 measures
A standard serum B12 test measures total cobalamin in the blood. That total includes cobalamin bound to holohaptocorrin as well as to holotranscobalamin — and only the holotranscobalamin fraction is actually available to your cells.
Because the test reports the total, a result can look adequate while the biologically active portion is not. As one laboratory policy review puts it, measuring total serum cobalamin “may mask true deficiency or falsely imply a deficient state.”
This is not a fringe criticism. A meta-analysis of serum B12 testing found sensitivity ranging from 13% to 75% and specificity from 45% to 100% across studies, depending on the reference standard used. The authors concluded that diagnosing conditions responsive to B12 supplementation “on the basis of cbl level alone cannot be considered a reliable approach.”
A sensitivity floor of 13% is not a rounding error. It means that under some conditions the test misses the large majority of cases.
The more sensitive marker
Methylmalonic acid (MMA) accumulates when B12 is functionally insufficient at the cellular level, which makes it a measure of whether the vitamin is doing its job rather than simply how much is circulating.
For detecting overt cobalamin deficiency, the sensitivity of elevated serum MMA is reported at greater than 95%. MMA is also described as being as sensitive as, but more specific than, homocysteine for this purpose.
The relevant clinical guidance is direct: MMA testing is indicated when B12 deficiency is suspected but the serum B12 level is normal or low-normal. That is exactly the situation many people with unexplained neuropathy are in — and exactly the situation where the follow-up test is most often skipped.
The metformin problem
This matters most for a group of people who are least likely to have it investigated.
Metformin interferes with B12 absorption. The UK’s MHRA advised in 2022 that the effect “affects up to one in 10 people” taking it, and recommended checking levels in patients with symptoms of deficiency and monitoring those at risk. A four-year trial of metformin at 850 mg three times daily found a mean decrease in B12 concentration of almost 20%.
The American Diabetes Association has recommended considering periodic assessment of B12 in patients on long-term metformin — specifically including those with peripheral neuropathy or anaemia.
Now consider the sequence that follows. A person with type 2 diabetes on long-term metformin develops burning feet. The symptom is consistent with diabetic neuropathy, the diagnosis is reasonable, and the workup often stops there. If a B12-related contributor is present, it is attributed to the diabetes and never separately identified — and if a serum B12 was drawn and came back mid-range, the possibility is considered closed.
Two conditions that look identical from the outside, and only one of them has been ruled out.
What this does and does not mean
It does not mean B12 explains most neuropathy. It usually does not. It does not mean supplementing B12 will resolve nerve symptoms — where nerve damage is established, correcting a contributor may slow or stop progression without reversing what has already occurred.
What it means is narrower and more useful: a normal serum B12 is weaker evidence than it is usually treated as, and there is a widely available, better-performing follow-up test for the exact scenario where the first one is least trustworthy.
If you have unexplained neuropathy and a normal B12 on file, reasonable questions are whether methylmalonic acid was ever measured, and — if you take metformin — whether B12 has been monitored at all.
You may also want to read what a nerve conduction study measures and what it cannot, which covers the same problem with a different test.
If you are in the Oklahoma City area, here is how we approach neuropathy evaluation, or you can start with a free phone consultation.
Reviewed by Alvin Philipose, DC, ICCP · 23 August 2026
This article is provided for general educational purposes only. It is not medical advice, not a diagnosis, and not an offer of treatment or a description of services. It has not been evaluated by the Food and Drug Administration. Do not start, stop or change any medication, including metformin, on the basis of this article — discuss it with the provider who prescribes it. Individual results vary and no outcome is guaranteed. If you are experiencing sudden weakness, difficulty breathing or swallowing, new bowel or bladder changes, or a wound or infection on your foot, seek emergency care.
Sources
- Vitamin B12 and Methylmalonic Acid Testing — medical policy review of serum B12 sensitivity and specificity, holotranscobalamin, and MMA performance. Policy document
- Vitamin B12 deficiency in long-term metformin use and clinician awareness: a scoping review protocol — citing MHRA (2022) and ADA guidance. medRxiv 2025.02.17.25322407
