Serum Folate Unit Converter
Serum Folate Unit Converter
Convert serum folate between ng/mL, µg/L and nmol/L. Two things this page will not do: pretend that the 2.266 factor is a molar conversion of what the assay actually measures, and pretend that a normal serum folate excludes deficiency.
Serum folate converter
Mass ⇄ molarSerum folate 6.0 ng/mL
The factor, and the molecule it belongs to
ng/mL = nmol/L ÷ 2.2655 = nmol/L × 0.4413
because 1 ng/mL is 1 × 10⁻⁹ g/mL = 10⁻⁶ g/L, and 10⁻⁶ ÷ 441.4 g/mol = 2.2655 nmol/L
- 441.4
- the molecular weight of folic acid, pteroylglutamic acid, C₁₉H₁₉N₇O₆ — recomputed from IUPAC standard atomic weights as 441.404. This is the molecule the factor is built on, and it is not the molecule the assay is measuring
- 2.2655
- the conversion the whole world uses, and a reporting convention rather than a molar conversion of the measurand. It is arithmetically exact for folic acid and 3.9% wrong for 5-methyltetrahydrofolate, which is what actually circulates
- ng/mL = µg/L
- identical. A nanogram in a millilitre is a microgram in a litre, so there is nothing to multiply. If your report switches between the two labels, nothing about the result has changed
- the honest caveat
- the factor is a convention that makes results comparable, and it should be used for that reason. It is not a count of the folate molecules in the sample, because those are a mixture of vitamers with molar masses from 441 to 473
- not red cell folate
- a different test on a different sample type, an order of magnitude higher, and with its own interval. The factor is the same 2.2655 because it is the same convention, but the numbers are not interchangeable
Worked example
Serum folate 6.0 ng/mL
6.0 × 2.2655 = 13.6 nmol/L
= 6.0 µg/L, since µg/L and ng/mL are numerically identical
Above the lower limits three of the four laboratories in the table below publish, and above ARUP's 5.9 ng/mL normal threshold — but only just, and a value this close to the limit in someone who has already started a supplement cannot exclude deficiency
Had the factor been computed from 5-methyltetrahydrofolate's own molar mass of 459.46 instead of folic acid's 441.4, the answer would be 13.1 nmol/L
The factor, and the four molecules it could have been built on
| Folate form | Formula | Molar mass | ng/mL → nmol/L would be |
|---|---|---|---|
| Folic acid (pteroylglutamic acid) — what the factor uses | C₁₉H₁₉N₇O₆ | 441.40 | × 2.2655 |
| 5-methyltetrahydrofolate — what actually circulates | C₂₀H₂₅N₇O₆ | 459.46 | × 2.1765 |
| Tetrahydrofolate — the bioactive form | C₁₉H₂₃N₇O₆ | 445.43 | × 2.2450 |
| 5-formyltetrahydrofolate | C₂₀H₂₃N₇O₇ | 473.44 | × 2.1122 |
The convention is at least internally exact: WHO’s own pairings
| WHO threshold | ng/mL | nmol/L as WHO prints it | × 2.2655 gives |
|---|---|---|---|
| Serum folate deficiency | < 3 | < 6.8 | 6.80 |
| Red cell folate deficiency | < 100 | < 226.5 | 226.55 |
| Red cell folate insufficiency — the population threshold for reduced neural tube defect risk | < 400 | 906 | 906.21 |
Four named laboratories, four different lower limits
| Laboratory | Method | Deficient | Indeterminate | Normal |
|---|---|---|---|---|
| Labcorp | Electrochemiluminescence immunoassay | < 3.0 ng/mL (< 6.8 nmol/L) | — | > 3.0 |
| Mayo Clinic Laboratories | Competitive-binding receptor assay | < 4.0 µg/L (< 9.1 nmol/L) | — | ≥ 4.0 |
| University of Iowa | Electrochemiluminescence immunoassay | < 2.2 ng/mL (< 5.0 nmol/L) | 2.2–4.1 | > 4.1 |
| ARUP Laboratories | Quantitative chemiluminescent immunoassay | ≤ 3.9 ng/mL (≤ 8.8 nmol/L) | 4.0–5.8 | ≥ 5.9 |
What a serum folate can and cannot settle
| Question | What serum folate does with it |
|---|---|
| Is this patient folate deficient right now? | Answers it, if the sample predates any treatment. Serum levels "decline after only a few days of negative folate balance", so a low result is an early and sensitive signal. |
| Has this patient been folate deficient over the past few months? | Cannot answer it. Serum folate tracks recent intake, and Mayo notes it "may be low in the presence of normal tissue stores". That question belongs to the red cell folate converter, which reflects the 120-day lifespan of the red cell. |
| The patient already started folic acid, or ate a fortified breakfast. Is a normal result reassuring? | No. A single folate-containing meal or a few days of supplementation can normalise it, so a normal serum folate in a treated or recently fed patient does not exclude the deficiency you are investigating. This is the commonest way the test misleads. |
| Is the anaemia due to folate or to B12? | Only half of it. Both produce a macrocytic anaemia and the two must be measured together — Mayo states they "ideally … should be measured simultaneously". Homocysteine rises in both; methylmalonic acid rises in B12 deficiency and not in folate deficiency, which is what separates them. See the homocysteine converter and the methylmalonic acid converter. |
| Can I just treat the low folate and move on? | Not before excluding B12 deficiency. Giving folate to a B12-deficient patient corrects the anaemia while the neurological damage continues, and StatPearls is explicit: B12 repletion must begin "before or alongside folic acid supplementation", because treating folate alone "may exacerbate the neurological complications associated with B12 deficiency". Check the vitamin B12 converter first. |
A conventional factor, a contested interval, and one genuine danger
The arithmetic is the least of it. Nanograms per millilitre and micrograms per litre are identical, and nanomoles per litre is the same figure multiplied by 2.2655 — a thousandth of a millionth of a gram per litre divided by 441.4 grams per mole, scaled to nanomoles. That factor is worth trusting for a reason better than tradition: the World Health Organization prints serum folate deficiency as below 3 ng/mL or below 6.8 nmol/L, red cell folate deficiency as below 100 ng/mL or 226.5 nmol/L, and the population insufficiency threshold as 400 ng/mL or 906 nmol/L. Multiply 3, 100 and 400 by 2.2655 and you get 6.80, 226.55 and 906.21. Three independent pairings from one authority, three matches.
And yet 441.4 is the molecular weight of folic acid, which is largely not what is in the sample. The CDC’s laboratory procedure manual for serum folate vitamers states that the primary circulating folate vitamer in serum is 5-methyltetrahydrofolate, lists its molar mass as 459.46 against folic acid’s 441.4, and gives conversion factors across the six vitamers it measures ranging from 2.112 to 2.266. The number the world converts with is the top of that range and belongs to the form that is least represented in an unsupplemented person. Convert 5-methyltetrahydrofolate on its own molar mass and the factor is 2.1765, so a folate of 6.0 ng/mL is 13.6 nmol/L by convention and 13.1 nmol/L by the chemistry of the actual measurand. Nothing clinical turns on four per cent, and the convention should be followed because comparability between laboratories is worth far more than notional accuracy. But a serum folate in nanomoles per litre is a nominal molar concentration computed on a surrogate molecule, not a count of the molecules present, and a page that says otherwise is misleading its reader about what units mean.
The reference interval is a worse problem than the factor, and it is why this page deliberately shows no interval and no bar. Four named reference laboratories on three closely related immunoassay methods place the lower limit of normal at 3.0, 4.0, 4.1 and 5.9 ng/mL. That is a twofold spread across exactly the range where results get queried: 4.5 ng/mL is comfortably normal for one laboratory and indeterminate for another. There is no meaningful upper limit to quote, because a high serum folate reflects supplements or fortified food and carries no clinical significance by itself. And the lower end is genuinely contested rather than merely variable — one analysis of tertiary-care patients argued for raising the limit from 3.0 to 13.0 ng/mL, on the grounds that 3.0 is the level that prevents megaloblastic anaemia while hyperhomocysteinaemia is prevented at around 4.0 and optimal homocysteine suppression may need about 7.0. Take the interval from your own report; it is the only one that was validated against the assay that produced the number.
What separates serum folate from the red cell folate converter is time. Folate is incorporated into the red cell during erythropoiesis and not exchanged afterwards, so a red cell folate reflects availability over the preceding four months. Serum folate reflects the last few days: it declines after only a few days of negative folate balance, which makes it an early and sensitive marker, and it can normalise after a single folate-containing meal or a short course of supplements, which makes a normal result in a treated patient close to uninterpretable. Mayo puts the same point the other way round, noting that serum folate "may be low in the presence of normal tissue stores". So the rule is simple and frequently broken: draw the sample before treatment starts, and do not accept a normal serum folate from someone who has already been given folic acid as evidence that they were never deficient. That is not an argument for ordering red cell folate routinely — it is less precise, needs a haematocrit-corrected lysate, and rarely changes management once serum folate and B12 are known — but it is the question the red cell test exists to answer.
The genuine danger on this page is not a unit error. A macrocytic anaemia can be due to folate deficiency, to B12 deficiency, or to both, and the two must be measured together — Mayo states they should ideally be measured simultaneously. Treating a low folate without checking B12 corrects the haematology while the neurology proceeds: folate replacement restores DNA synthesis and the anaemia improves, but B12’s separate role in myelin maintenance is untouched, and subacute combined degeneration of the cord can be precipitated or worsened. StatPearls is unambiguous that B12 repletion must be started before or alongside folic acid, because treating folate alone may exacerbate the neurological complications of B12 deficiency. Measure the vitamin B12 converter on the same sample. Where both results are equivocal, homocysteine rises in either deficiency while methylmalonic acid rises in B12 deficiency and not in folate deficiency, which is what the homocysteine converter and the methylmalonic acid converter are for.
Frequently asked questions
How do I convert serum folate from ng/mL to nmol/L?
Multiply by 2.266. A serum folate of 6.0 ng/mL is 13.6 nmol/L, 4.0 ng/mL is 9.1 nmol/L and 3.0 ng/mL is 6.8 nmol/L. To go back, divide by 2.266 or multiply by 0.4413. The factor comes from the molecular weight of folic acid, 441.4 g/mol, and the WHO prints the 3 ng/mL deficiency threshold as 6.8 nmol/L, which is exactly what it gives.
Is µg/L the same as ng/mL for folate?
Yes, exactly and always. One nanogram in a millilitre is one microgram in a litre, because the prefix and the volume both change a thousandfold and cancel. A laboratory that reports 6.0 µg/L and one that reports 6.0 ng/mL have measured the same thing and there is nothing to convert between them.
Why is the 2.266 factor not a true molar conversion?
Because it uses folic acid’s molar mass of 441.4 g/mol, and folic acid is not mostly what circulates. The CDC’s own method documentation states that the primary circulating folate vitamer in serum is 5-methyltetrahydrofolate, whose molar mass is 459.46, and gives factors for the six vitamers it measures ranging from 2.112 to 2.266. Converting on 5-methyltetrahydrofolate would give 2.1765, about 4% lower. Use 2.266 anyway, because it is the shared convention — just do not read the nmol/L figure as a count of molecules.
What is a normal serum folate?
There is no single answer, which is why this page shows no reference interval. Four named reference laboratories give lower limits of 3.0, 4.0, 4.1 and 5.9 ng/mL on closely related immunoassays — a twofold spread — and ARUP additionally reports an indeterminate band of 4.0 to 5.8. A high result carries no clinical significance, so no upper limit is worth quoting. Use the interval printed beside your own result.
Does a normal serum folate exclude folate deficiency?
No, and this is the commonest way the test misleads. Serum folate tracks recent intake and declines within a few days of negative folate balance, so it also recovers within a few days of a supplement or a fortified meal. A normal result in a patient who has already started folic acid, or who has eaten well since admission, does not exclude the deficiency being investigated. Red cell folate reflects the preceding four months and is the test for that question.
Why must vitamin B12 be checked before treating folate deficiency?
Because folate replacement corrects the anaemia of B12 deficiency without correcting the neurological damage, and can allow subacute combined degeneration of the cord to progress or worsen. StatPearls states that in concurrent B12 deficiency, repletion must be initiated before or alongside folic acid. Measure B12 on the same sample; where both are equivocal, a raised methylmalonic acid points to B12 rather than folate.
What is the difference between serum folate and red cell folate?
Serum folate reflects the last few days of intake and is the more precise assay; red cell folate reflects the preceding 120 days, the lifespan of the red cell, and is the more stable measure but a less reliable measurement, requiring a haematocrit-corrected whole-blood lysate. Red cell concentrations are roughly an order of magnitude higher, so a value plausible for one is obviously wrong for the other — a useful check when a result arrives without its sample type.
Related calculators
References
- Centers for Disease Control and Prevention, National Health and Nutrition Examination Survey. Laboratory Procedure Manual: Folate Vitamers, Serum. "The primary circulating folate vitamer in serum is 5-methyltetrahydrofolate (5-methylTHF)"; molar masses 5-methylTHF 459.46, folic acid (PGA) 441.4, THF 445.43, 5-formylTHF 473.44; conversion factors "ranging from 2.112 to 2.266".
- World Health Organization. Guideline: Optimal Serum and Red Blood Cell Folate Concentrations in Women of Reproductive Age for Prevention of Neural Tube Defects. NCBI Bookshelf NBK294182. Deficiency "<3 ng/mL (<6.8 nmol/L) in serum and <100 ng/mL (<226.5 nmol/L) in red blood cells"; insufficiency threshold "red blood cell folate below 400 ng/mL [906 nmol/L]".
- ARUP Laboratories. Folate, Serum, test 0070070. "Less than or equal to 3.9 ng/mL: Deficient / 4.0-5.8 ng/mL: Indeterminate / Greater than or equal to 5.9 ng/mL: Normal"; quantitative chemiluminescent immunoassay; protect from light.
- Mayo Clinic Laboratories. Folate, Serum (FOL, test 9198). ">= 4.0 mcg/L" normal; competitive-binding receptor assay; "Serum folate levels typically fall within a few days after dietary folate intake is reduced" and "may be low in the presence of normal tissue stores"; B12 and folate "ideally … should be measured simultaneously".
- Labcorp. Folate (Folic Acid), test 002014. Reference interval >3.0 ng/mL by electrochemiluminescence immunoassay; "A serum folate concentration <3 ng/mL is considered to represent clinical deficiency by the World Health Organization".
- Clinical utility of serum folate measurement in tertiary care patients: argument for revising reference range for serum folate from 3.0 ng/mL to 13.0 ng/mL. Pract Lab Med. 2015. "Serum levels of folate decline after only a few days of negative folate balance"; hyperhomocysteinaemia prevented at ≥4.0 ng/mL, optimal suppression around 7.0 ng/mL.
- Folic acid deficiency. In: StatPearls. NCBI Bookshelf NBK535377. "In cases of concurrent vitamin B12 deficiency, it is essential to initiate B12 repletion before or alongside folic acid supplementation … Treating folate deficiency alone … may exacerbate the neurological complications associated with B12 deficiency."
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.