Red Cell Folate Unit Converter
Red Cell Folate Unit Converter
Convert red cell folate between ng/mL, µg/L and nmol/L, and see why the result reflects the preceding four months rather than the last meal.
Red Cell Folate converter
Mass ⇄ molarRed cell folate 250 ng/mL
Formula and conversion factor
ng/mL = nmol/L ÷ 2.266
- 2.266
- derived from the molecular weight of folic acid, 441.4 Da
- µg/L
- numerically identical to ng/mL — no conversion is needed between them
- haematocrit
- the result is calculated from a whole-blood lysate and the patient's haematocrit, so an error in either propagates into the reported folate
Worked example
Red cell folate 250 ng/mL
250 × 2.266 = 566 nmol/L
= 250 µg/L, since µg/L and ng/mL are numerically identical
Within the 140–628 ng/mL interval shown here
Reference interval converted
| ng/mL (µg/L) | nmol/L | |
|---|---|---|
| Lower limit, widely used | 140 | 317 |
| Worked example | 250 | 566 |
| Upper limit, widely used | 628 | 1,423 |
| WHO population threshold for reduced neural tube defect risk | > 400 | > 906 |
Red cell folate compared with serum folate
| Serum folate | Red cell folate | |
|---|---|---|
| Reflects | Recent intake, over days | Folate status over the preceding 120 days |
| Affected by a recent meal | Yes — can normalise after one folate-containing meal | No |
| Falls in B12 deficiency | No (may be raised) | Yes, through the methyl-folate trap |
| Analytical precision | Better | Worse — needs a haematocrit-corrected lysate |
What red cell folate measures, and why fewer laboratories offer it
Red cell folate is converted to nmol/L by multiplying ng/mL by 2.266, a factor derived from the molecular weight of folic acid, 441.4 Da. Results reported in µg/L need no conversion at all, since µg/L and ng/mL are numerically identical. Red cell concentrations are an order of magnitude higher than serum concentrations, so a value that looks plausible for one is obviously wrong for the other — a useful check when a result arrives without its unit or its sample type.
The test exists because folate is incorporated into the erythrocyte during erythropoiesis and is not exchanged afterwards. The measured concentration therefore reflects folate availability over the preceding 120 days, the lifespan of the red cell, and is unaffected by what the patient has just eaten. Serum folate, by contrast, tracks recent intake and can normalise after a single folate-containing meal or a few days of supplementation, which is precisely why a normal serum folate in a patient who has already started treatment cannot exclude deficiency.
The important trap is that red cell folate falls in vitamin B12 deficiency as well as in folate deficiency. Methylenetetrahydrofolate reductase converts folate to 5-methyltetrahydrofolate, and B12 is required by methionine synthase to release that methyl group and regenerate tetrahydrofolate for polyglutamation. Without B12 the folate is stranded as monoglutamate, leaks out of the cell and is not retained — the methyl-folate trap. A low red cell folate therefore does not by itself prove folate deficiency, and B12 must be measured and corrected first, since giving folate alone to a B12-deficient patient can precipitate or worsen subacute combined degeneration of the cord.
The assay itself is the other limitation. It requires a whole-blood haemolysate and a haematocrit-corrected calculation, so an error in the haematocrit propagates directly into the reported value, and between-platform variation of 30% or more has been documented for the same sample. That imprecision, together with the lack of evidence that it changes management once serum folate and B12 are known, is why many laboratories have withdrawn red cell folate from routine use and restricted it to specific enquiries such as population folate status assessment.
Frequently asked questions
How do I convert red cell folate from ng/mL to nmol/L?
Multiply by 2.266, derived from the molecular weight of folic acid, 441.4 Da. A red cell folate of 250 ng/mL is 566 nmol/L. Results in µg/L are numerically the same as ng/mL.
What is the difference between serum and red cell folate?
Serum folate reflects recent intake and can normalise after a single folate-containing meal. Red cell folate reflects folate status over the preceding 120 days, the lifespan of the red cell, and is not affected by the last meal.
Can red cell folate be low in vitamin B12 deficiency?
Yes. B12 is needed to release the methyl group from 5-methyltetrahydrofolate so folate can be retained inside the cell as a polyglutamate — the methyl-folate trap. A low red cell folate therefore does not by itself prove folate deficiency, and B12 must be checked.
Why have some laboratories stopped offering red cell folate?
It requires a haematocrit-corrected calculation from a whole-blood lysate and is considerably less precise than serum folate, with large between-platform differences on the same sample, and it rarely changes management once serum folate and B12 are known.
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References
- Institute of Medicine. Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline. National Academies Press; 1998.
- World Health Organization. Serum and Red Blood Cell Folate Concentrations for Assessing Folate Status in Populations. Vitamin and Mineral Nutrition Information System. Geneva: WHO; 2015.
- Galloway M, Rushworth L. Red cell or serum folate? Results from the National Pathology Alliance benchmarking review. J Clin Pathol. 2003;56(12):924–926.
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.