Vitamin B12 pg/mL to pmol/L Converter

Vitamin B12 in two units: the short answer

To convert vitamin B12 from pg/mL to pmol/L, multiply by 0.738. To convert pmol/L back to pg/mL, divide by 0.738 (or multiply by 1.355). A B12 level of 200 pg/mL is therefore about 148 pmol/L, and 300 pmol/L is about 407 pg/mL. Because pg/mL equals ng/L, a report in ng/L can be read as if it were in pg/mL. The converter above handles both directions and shows where your value falls against commonly used thresholds.

B12 is a test where the unit can change the interpretation dramatically. "200" means deficient in pg/mL but comfortably normal in pmol/L. If you compare your result with a threshold from a website or guideline written in the other unit, it is easy to draw the wrong conclusion – which is exactly what this page aims to prevent. It also explains why a "normal" B12 does not always rule out deficiency.

What vitamin B12 is and what the test measures

Vitamin B12 (cobalamin) is needed to make red blood cells, to keep nerves healthy, and to make DNA. It is found naturally only in animal foods – meat, fish, eggs and dairy – and in fortified foods and supplements. Absorption is complex: B12 is released from food by stomach acid, binds to intrinsic factor made by the stomach lining, and is absorbed in the last part of the small intestine (the terminal ileum). The body stores several years' worth, mostly in the liver, so deficiency usually develops slowly.

The standard blood test measures total serum B12. About 70–80% of this is bound to a carrier protein called haptocorrin and is not available to cells. The remaining 20–30% is bound to transcobalamin; this fraction, called holotranscobalamin (holoTC) or "active B12", is what cells can take up. Some laboratories offer an active B12 test, with its own units and thresholds (usually pmol/L).

pg/mL and pmol/L explained

• pg/mL (picograms per milliliter) – used mostly in the United States. ng/L is identical.
• pmol/L (picomoles per liter) – the SI unit, used in the UK, Europe, Australia and elsewhere.

Why the factor is 0.738

Cyanocobalamin has a molar mass of about 1,355 g/mol. A concentration of 1 pg/mL is 1,000 pg per liter, which is 1,000 ÷ 1,355 = 0.738 pmol/L. Conversely, 1 pmol/L = 1.355 pg/mL.

How to convert vitamin B12 step by step

1. Check the unit: pg/mL, ng/L or pmol/L.
2. For pg/mL (or ng/L) to pmol/L, multiply by 0.738.
3. For pmol/L to pg/mL, divide by 0.738.
4. Round to whole numbers.

Worked examples

• 150 pg/mL × 0.738 = 111 pmol/L
• 200 pg/mL × 0.738 = 148 pmol/L – the classic deficiency cut-off in US units.
• 300 pg/mL × 0.738 = 221 pmol/L – the upper edge of the borderline zone.
• 500 pg/mL × 0.738 = 369 pmol/L
• 148 pmol/L ÷ 0.738 = 201 pg/mL
• 180 pmol/L ÷ 0.738 = 244 pg/mL – a level many UK laboratories flag for further assessment.
• 600 pmol/L ÷ 0.738 = 813 pg/mL

Mental shortcut

Multiply pg/mL by three-quarters, or multiply pmol/L by 4 and divide by 3. For 400 pg/mL: 400 × 0.75 = 300 pmol/L (exact: 295).

Vitamin B12 conversion chart

Vitamin B12 levels: what the numbers mean

The thresholds above are the ones most widely used in clinical practice:

• Below 200 pg/mL (148 pmol/L) – deficiency likely, particularly with symptoms or anaemia.
• 200–300 pg/mL (148–221 pmol/L) – borderline or "indeterminate". Some people in this range are deficient at the tissue level, and further testing is recommended.
• Above 300 pg/mL (221 pmol/L) – deficiency unlikely in most people.

Laboratory reference ranges vary considerably; a typical upper limit is around 900 pg/mL (664 pmol/L), but some laboratories use different values. The British Society for Haematology guideline emphasises that no single cut-off reliably separates deficient from non-deficient people and that clinical features should guide testing and treatment.

Why the test can mislead

Total serum B12 is an imperfect test:

• False normal or high results can occur in people who are deficient at the cell level, for example with liver disease, some blood cancers, or antibodies to intrinsic factor that interfere with certain assays in pernicious anaemia. The BSH guideline specifically warns that a normal B12 does not exclude deficiency when there is a strong clinical suspicion.
• False low results can occur in pregnancy, with folate deficiency, with haptocorrin deficiency, and with oral contraceptives, without true tissue deficiency.

Confirmatory tests

When the result is borderline or does not fit the clinical picture, these tests help:

• Methylmalonic acid (MMA) – rises when B12 is lacking at the cell level. A raised MMA (often above about 0.4 µmol/L, depending on the laboratory) supports deficiency, but MMA also rises with kidney impairment.
• Homocysteine – rises in both B12 and folate deficiency, and with kidney disease and some genetic variants. Less specific than MMA.
• Holotranscobalamin (active B12) – a low holoTC (often below about 25–35 pmol/L, depending on the laboratory) supports deficiency.
• Intrinsic factor antibodies – positive in about half of people with pernicious anaemia and highly specific for it.

Causes of B12 deficiency

• Pernicious anaemia – an autoimmune condition in which the stomach stops making intrinsic factor. It is the most common cause of severe deficiency in many countries and requires lifelong B12, usually by injection or high-dose tablets.
• Diet – vegan diets without supplements or fortified foods, and very restricted diets.
• Stomach and gut conditions – gastrectomy or weight-loss surgery, atrophic gastritis, coeliac disease, Crohn's disease affecting or removal of the terminal ileum.
• Medicines – long-term metformin use (guidelines suggest periodic B12 checks), long-term proton pump inhibitors and H2 blockers, and nitrous oxide (including recreational use, which can cause severe neurological damage even when blood B12 looks normal).
• Ageing – reduced stomach acid in older adults impairs release of B12 from food.

Symptoms of B12 deficiency

Symptoms develop gradually and include fatigue, breathlessness, pale or slightly yellow skin, a sore red tongue and mouth ulcers. Nervous system effects can occur even without anaemia: tingling or numbness in the hands and feet, problems with balance and walking, memory problems, confusion and mood changes. Some neurological damage can become permanent if treatment is delayed, which is why borderline results with neurological symptoms are taken seriously.

High vitamin B12

A high B12 is most often due to supplements or injections and is usually harmless, since excess is excreted. A persistently high B12 without supplements can sometimes reflect liver disease, kidney disease or certain blood disorders such as myeloproliferative neoplasms, and may warrant a check with a doctor.

Recommended intake

The US recommended dietary allowance for adults is 2.4 µg per day, rising to 2.6 µg in pregnancy and 2.8 µg during breastfeeding. Adults over 50 are advised to get most of their B12 from fortified foods or supplements, because the crystalline form does not depend on stomach acid for release. Vegans need a reliable supplement or fortified foods. Note that dose units (µg or mcg) are completely different from the blood test units (pg/mL or pmol/L).

Treatment and follow-up

B12 deficiency is treated with either injections (intramuscular hydroxocobalamin in the UK, cyanocobalamin in the US) or high-dose oral B12, depending on the cause and the severity:

• Pernicious anaemia and malabsorption – traditionally treated with injections. High-dose oral B12 (1,000–2,000 µg daily) can also work because about 1% is absorbed passively without intrinsic factor, and some guidelines accept it as an alternative for people without neurological symptoms.
• Dietary deficiency – oral supplements are usually enough, followed by long-term supplementation or fortified foods.
• Neurological symptoms – treated urgently with more frequent injections at first.

Blood counts usually improve within one to two weeks of starting treatment, and anaemia corrects over about eight weeks. Once treatment has started, retesting B12 is generally not useful for monitoring injection therapy, because levels are high after each dose; the response is judged by symptoms and blood counts instead. If folate is also low, B12 should be replaced first or at the same time, because treating folate alone can worsen nerve damage.

Vitamin B12 in pregnancy and infants

Total B12 falls during pregnancy, and pregnant women may have results below non-pregnant reference ranges without being deficient. Nevertheless, true deficiency in pregnancy matters, because babies born to deficient mothers – especially exclusively breastfed babies of vegan mothers without supplementation – can develop deficiency in the first months of life, with poor feeding, developmental delay and anaemia. Pregnant and breastfeeding women on vegan or vegetarian diets should take a B12 supplement.

Converting active B12 (holotranscobalamin)

Active B12 is usually reported only in pmol/L, so conversion is seldom needed. Its thresholds are much lower than total B12 – often below about 25–35 pmol/L for deficiency and above about 50 pmol/L for normal – so do not compare an active B12 result with total B12 thresholds.

Metformin and vitamin B12

Metformin, the most widely used medicine for type 2 diabetes, reduces B12 absorption in the ileum. Long-term use is associated with lower B12 levels, and the risk increases with dose and duration. The American Diabetes Association recommends periodic measurement of B12 in people taking metformin long term, especially those with peripheral neuropathy or anaemia. This matters because the tingling and numbness of B12 deficiency can be mistaken for diabetic neuropathy. In the UK, NICE's 2024 guideline also lists long-term metformin as a risk factor that should prompt testing when symptoms are present.

Comparing B12 results across laboratories

B12 assays are not fully standardised, and different platforms can give results that differ by 20% or more. Consider a person who had 210 pmol/L at a European laboratory and later 240 pg/mL at a US laboratory. Converted, 210 pmol/L is 285 pg/mL, and 240 pg/mL is 177 pmol/L. Although the second number looks higher, the level actually fell by about 15% – or the two assays simply disagree. In this kind of situation, a confirmatory test (MMA or active B12) is more informative than chasing small changes in total B12.

What affects vitamin B12 results

• Recent supplements or injections – an injection can raise blood levels for weeks; testing soon after treatment does not reflect stores.
• Pregnancy – total B12 falls progressively, partly due to blood dilution and lower carrier protein, without necessarily meaning deficiency.
• Folate status – folate deficiency can lower measured B12, and high folate intake can mask the anaemia of B12 deficiency while nerve damage continues.
• Oral contraceptives and HRT – may lower total B12 by reducing haptocorrin.
• Kidney disease – raises MMA and homocysteine, complicating interpretation.
• Assay interference – intrinsic factor antibodies can produce falsely normal results on some platforms.

Who is at higher risk of B12 deficiency

Testing is worth considering in people with symptoms who also belong to a higher-risk group:

• adults over 65, because reduced stomach acid impairs absorption from food
• people with autoimmune conditions such as type 1 diabetes, autoimmune thyroid disease or vitiligo, which are associated with pernicious anaemia
• people with a family history of pernicious anaemia
• vegans and vegetarians who do not take supplements or eat fortified foods
• people who have had stomach or bowel surgery, including weight-loss surgery
• people with Crohn's disease, coeliac disease or other causes of malabsorption
• long-term users of metformin, proton pump inhibitors or H2 blockers
• people who use nitrous oxide recreationally or are repeatedly exposed to it

B12 deficiency and nitrous oxide

Nitrous oxide inactivates B12 inside the body by oxidising its cobalt atom. Repeated recreational use can cause severe nerve and spinal cord damage, sometimes with a normal or only mildly low blood B12, because the vitamin is present but inactive. MMA and homocysteine are usually raised. Treatment involves stopping nitrous oxide and urgent B12 replacement; recovery may be incomplete.

When to see a doctor

See a doctor if your B12 is below 200 pg/mL (148 pmol/L), or in the borderline range with symptoms such as tingling, numbness, balance problems, memory changes or anaemia. Seek prompt care if you have rapidly worsening numbness, weakness or difficulty walking, which can indicate damage to the spinal cord. People taking metformin long term, those who have had stomach or bowel surgery, and vegans not taking supplements should ask about periodic testing.