B12 levels

This page explains how to read a B12 result; it does not recommend personal treatment or doses. General information is in the vitamin B12 guide, and symptoms are covered on the deficiency symptoms page.

What should B12 levels be? Equivalents in pg/mL and pmol/L

On a laboratory report, B12 can be written in two different units: pg/mL or pmol/L. The conversion is approximately 1 pg/mL = 0.738 pmol/L; the equivalents given by NIH are consistent with this (200 pg/mL = 148 pmol/L). The table below summarizes the thresholds reported by NIH and the Turkish Hematology Association; it is not a diagnosis on its own. ^{[1] [3]}

These numbers are laboratory-specific; in the same person, different methods can give different results. If the report lists the laboratory’s own reference range, the comparison is made against it first.

Why does every laboratory have a different range?

The British Society for Haematology (BSH) guideline reports that, because of the variety of methods used and technical problems, precise cutoffs defining clinical and subclinical deficiency could not be established, so each site should establish its own reference ranges. The same guideline emphasizes that there is no “gold standard” test defining deficiency and that the clinical picture is the most important factor. ^[4]

An example from Türkiye: in a study in Konya that evaluated 1,055 healthy people aged 0–24 years and 54 cord blood samples, the 2.5th–97.5th percentile range was 127–590 pg/mL in the whole group. The authors noted that reference values reported in other studies were higher and that comparison between populations can be misleading. This finding is for children and young people; it cannot be applied to adults. ^[5]

What are MMA, homocysteine and holotranscobalamin used for in a borderline result?

Serum B12 is the first-line test; when it remains unclear, indirect (functional) markers come into play. ^{[1] [2] [4]}

BSH says that if the test result and strong clinical signs of deficiency are discordant, treatment should not be delayed in order to prevent nerve damage. This decision rests with the physician. ^[4]

Are these values read the same in pregnancy, older age and kidney disease?

Not always. According to NIH, serum B12 levels tend to fall in pregnancy, sometimes dropping below normal, and usually return to normal after delivery. MMA rises in kidney failure and tends to be higher in older adults; homocysteine is also affected by kidney function. For this reason, in special circumstances the same number can carry different meanings, and interpretation should be left to the physician. ^[1]

What does a high B12 level mean?

There is no single threshold for “high B12”; studies use different limits. In a region of Denmark, among 333,667 people who had B12 measured, 6% had a level above the upper reference limit (601 pmol/L and above). This shows that high results are not rare. ^[6]

The most common cause: supplements and injections

The most common cause of a high level is a B12 supplement or injection; in a study in the Netherlands of 72 patients with B12 above 1,476 pmol/L, the elevation could be explained by supplement use in 79% of patients. Multivitamins containing B12, B-complex and fortified products can also contribute to this result. NIH reports that, because of its low toxicity potential, no tolerable upper intake level has been set for B12; a supplement-related rise is therefore generally regarded not as a separate hazard but as an expected result. Informing the physician that a supplement or injection is being used changes the interpretation of the result. ^{[1] [8]}

Persistent elevation without supplements

Among the causes of persistent, unexplained high B12 in a person not taking supplements are liver disease, kidney disease and myeloid blood diseases. ^[8] In this case, the physician can also assess whether the result is real or laboratory-related: “macro-B12” is a biologically inactive complex that B12 forms with transcobalamin and immunoglobulins, and because its clearance is reduced it can raise the measured level. In one study, macro-B12 was detected in 24% of 72 people with levels above 1,476 pmol/L; a polyethylene glycol (PEG) precipitation test can help demonstrate it. ^[8]

High B12 and the relationship with cancer

In Danish registry data, among 333,667 people not receiving B12 treatment, the risk of a later cancer diagnosis rose as B12 levels rose, and the risk was greatest in the first year of follow-up: the first-year SIR was 3.44 for 601–800 pmol/L and 6.27 for above 800 pmol/L. Over the whole follow-up, the SIR in these two groups was 1.61 and 2.38, and after the first year 1.09 and 1.24. In people with high B12, the increase was particularly marked for blood cancers and for cancers associated with smoking and alcohol. The SIR here is a comparison with the general population; in people in the normal range (200–600 pmol/L), the first-year SIR was also 2.04. This suggests that people for whom B12 testing is requested are already a group being evaluated for a complaint. The authors noted that this may have clinical consequences for the interpretation of high B12 levels. ^[6]

This is not evidence that “B12 causes cancer”: the study is observational; a possible interpretation is that a high level may be a marker of a disease not yet diagnosed, but causality cannot be shown with this study. A 2025 meta-analysis covering 28 studies and 69,610 participants also did not show that high B12 increases the risk of all-cause death overall in adults; a borderline increase was found only in the subgroups with chronic disease (RR 1.40) and hospitalized patients (RR 1.57), and these subgroup differences were not significant in meta-regression. ^{[6] [7]}

What is done when a result is high?

When discussing the result with a physician, the following information helps: whether a B12 supplement, B-complex, energy drink or injection was taken in recent weeks, known liver and kidney diseases, and the blood count. If a supplement is being used and the result fits it, further investigation is often not needed; if the result stays high without a supplement, repeat measurement and, if necessary, additional evaluation are planned. This guide does not determine which test is done and when.

Practical points when interpreting the result

• The unit (pg/mL or pmol/L) and the laboratory’s own reference range are considered first. ^{[1] [4]}
• The result gains meaning together with symptoms, medications used (metformin, PPIs, H2 blockers) and diet. ^[1]
• A borderline value does not establish a diagnosis on its own; tests such as MMA and homocysteine are decided by the physician. ^{[1] [4]}
• A measurement taken after starting a supplement or injection does not reflect the baseline status.