The "serum folates" line on your blood test is below normal, sometimes marked in bold. What does this measurement show, at what level do we speak of deficiency, and what should you do next? The answer is straightforward, provided you know what this number reflects, and what it does not.
This guide explains what folates are, the difference between serum and red blood cell folates, published thresholds, causes to check, including certain medications, what a low level can lead to, why vitamin B12 should be checked before supplementing, and how long it takes to raise your folate levels.
What the measurement shows : serum folates mainly reflect your intake from the past few days; red blood cell folates, your reserves from the past few months. Published reference points: below 7 nmol/L, approximately 3 ng/mL, risk of deficiency; below 10 nmol/L, approximately 4.4 ng/mL, possible deficiency. Thresholds depend on the technique: your laboratory's range is definitive, and the result should be read with the complete blood count and vitamin B12.
What to do : identify the cause—low intake, alcohol, celiac disease, pregnancy, medications such as methotrexate, trimethoprim, antiepileptic drugs or sulfasalazine, smoking; test B12 before supplementing ; then correct through diet and supplementation with folic acid or methylfolate. Plasma folates reach a plateau around 3 months, red blood cell folates in 6 to 12 months; the safety limit is 1,000 µg per day for added forms.
- Folates, folic acid, methylfolate: what are we talking about
- Serum or red blood cell folates: what each test measures
- Reading your result: the thresholds
- Why folates are low: causes to check
- What a low level can lead to
- Check B12 before supplementing
- Raising your folates: diet, folic acid or methylfolate
- How long it takes, and when to retest
- The plan, in order
- Frequently Asked Questions
Folates, folic acid, methylfolate: what are we talking about
<<<15>>> Folates folates are the natural forms of vitamin B9, found in leafy green vegetables, legumes, liver, eggs and fruits. They are essential for DNA production and therefore for cells that renew quickly, primarily red blood cells.Folic acid is the stable synthetic form found in supplements and fortified foods: the body must reduce it before using it. Methylfolate, or 5-methyltetrahydrofolate, is the active form: according to a pharmacokinetic review, it is the only form of folate that normally circulates in the blood.
At equivalent doses, the bioavailability of folic acid and methylfolate is comparable. In a 24-week trial in 144 women of childbearing age, 416 µg of methylfolate raised red blood cell folates more than 400 µg of folic acid; and in a 2026 trial in 110 pregnant women, methylfolate in glucosamine salt form raised erythrocyte folates as much as folic acid, with comparable homocysteine reduction.
Serum or erythrocyte folates: what each test measures
In 1,673 Chinese women, of whom 1,108 took folic acid for six months at different doses, researchers correlated plasma folates and red blood cell folates. Plasma folates reflect very recent intake; those in red blood cells reflect long-term intake and liver reserves, and take approximately six to twelve months to reach equilibrium. The level of plasma folates corresponding to the threshold for preventing neural tube defects was 25.5 nmol/L, and 34.6 nmol/L in the case of vitamin B12 deficiency.
Chen MY, Rose CE, Qi YP, et al. Am J Clin Nutr 2019;109(5):1452-1461. DOI: 10.1093/ajcn/nqz027
British hematology guidelines make serum testing the first-line examination, with diagnostic value equivalent to that of erythrocyte folates. They place serum vitamin B12 first among the other vitamin to check, supplemented by methylmalonic acid in case of doubt.
Because it reflects the previous days, serum testing varies with what was recently eaten: a folate-rich meal or a supplement taken shortly before the blood draw causes it to rise, a few days of poor nutrition cause it to fall. Hemolysis of the sample, when red blood cells rupture in the tube, also skews the result upward, since they contain much more folates than serum. And during pregnancy, blood dilution lowers serum folates.
Reading your result: the thresholds
| Test | Threshold | Significance | Source |
|---|---|---|---|
| Serum folates | Less than 7 nmol/L, approximately 3.1 ng/mL | Risk of deficiency, with possible megaloblastic anemia | NHANES Data, 2016 |
| Serum folates | Less than 10 nmol/L, approximately 4.4 ng/mL | Possible deficiency, homocysteine beginning to rise | NHANES Data, 2016 |
| Red blood cell folates | Less than 305 nmol/L, approximately 135 ng/mL | Risk of deficiency | NHANES Data, 2016 |
| Red blood cell folates | Less than 340 nmol/L, approximately 150 ng/mL | Possible deficiency | NHANES Data, 2016 |
| Red blood cell folates | More than 906 nmol/L, or 400 ng/mL | Level associated with the best prevention of neural tube defects at the population level | WHO, 2015 |
Two clarifications change the interpretation. First, the thresholds depend on the measurement technique: in American data, applying thresholds established with another technique changed the proportion of folate insufficiency from 3% to 39% of the population. The reference range printed on your lab report, specific to the laboratory's method, is therefore the first reference. Second, the World Health Organization does not set a threshold for serum folates during pregnancy: it bases its reasoning on red blood cell folates, and recommends re-evaluation at least six to twelve months after an intervention.
Laboratories report folates in nmol/L or ng/mL. The WHO benchmark of 906 nmol/L for 400 ng/mL provides the conversion factor: 1 ng/mL corresponds to approximately 2.27 nmol/L. A result of 5 ng/mL therefore equals approximately 11 nmol/L.
Why folate levels are low: causes to check
| Cause | What happens | Source |
|---|---|---|
| Low intake | Few green vegetables, legumes, and fruits; smokers consume less | 2011 Review |
| Alcohol | Reduced intake, reduced absorption, reduced liver storage, increased urinary losses | 2013 Review |
| Celiac disease | Malabsorption; one quarter of untreated adults anemic, folate deficiency among the causes | Cohort of 572 patients, 2022 |
| Pregnancy | Increased needs and blood dilution; serum folates divided by more than two without supplementation | Randomized trial, 2013 |
| Methotrexate, trimethoprim, pyrimethamine | Block the enzyme that activates folates | 1985 Review |
| Antiepileptic drugs | Six medications associated with lower folate levels in 2,730 patients | 2011 study |
| Sulfasalazine | Inhibits folate absorption | 1981 study |
| MTHFR 677TT variant | Serum folates 13% and erythrocyte folates 16% lower | Meta-analysis of 40 studies, 2015 |
The most common cause remains diet, especially since serum levels reflect only the preceding days. Our ranking of foods rich in vitamin B9 provides nutrient content and the effect of cooking. Alcohol impacts every stage, from intake to elimination. Anemia with folate deficiency in an adult with a varied diet should prompt investigation for malabsorption, including celiac disease.
On the medication side, methotrexate is the classic example: it works by blocking the enzyme that activates folates. In patients with rheumatoid arthritis, folic or folinic acid prescribed with it reduces digestive side effects by 26% and liver enzyme elevations by 77%, without diminishing its efficacy, according to the Cochrane review. Under antiepileptic drugs, three months of supplementation normalized levels in 95% of the 141 patients who had low folates or B12. In all cases, supplementation should be decided with the doctor prescribing the treatment.
What low levels can cause
When deficiency develops, red blood cell production slows and they become too large: this ismegaloblastic anemia, with elevated mean corpuscular volume on the complete blood count, fatigue and shortness of breath with exertion. Neurological symptoms point more toward vitamin B12 deficiency rather than folate deficiency.
Even before anemia,homocysteine, an amino acid that folates help recycle, rises in the blood. A meta-analysis of 25 trials and 2,596 people shows that folic acid lowers it by 13% at 0.2 mg per day, by 20% at 0.4 mg, and by 23-25% at 0.8 mg or higher; vitamin B12 adds another 7%.
Five trials and 7,391 women evaluated folate supplementation around conception. It reduced the risk of neural tube defects by 69%, and the risk of recurrence in women already affected by 66%. The dose, starting at 400 µg per day, did not modify the effect.
De-Regil LM, Peña-Rosas JP, Fernández-Gaxiola AC, Rayco-Solon P. Cochrane Database Syst Rev 2015;(12):CD007950. DOI: 10.1002/14651858.CD007950.pub3
This is why planning a pregnancy warrants supplementation without waiting for test results, and why low folates during pregnancy should be reported promptly to the doctor or midwife. Our guide tofolic acid during pregnancy details the timeline.
Check B12 before supplementing
Folates and vitamin B12 work together, and their deficiencies cause the same anemia. The known pitfall is this: at high doses, above 5 mg per day, folic acid can correct the anemia from B12 deficiency while nerve damage continues to progress, and a 2024 review associates low B12 with elevated folates with poorer cognitive performance. In elderly Americans with low B12, serum folates above 59 nmol/L were associated with a 3.1-fold increased risk of anemia and a 2.6-fold increased risk of cognitive impairment.
Hence two rules. Measure serum B12, supplemented if needed with methylmalonic acid, before supplementing with folates. And respect the safety limit set by EFSA, 1,000 µg per day in adults, including during pregnancy, for folic acid and methylfolate salts combined. Methylfolate could reduce this masking risk, according to pharmacokinetic reviews, but it does not replace B12 testing.
Tingling or numbness in the hands and feet, balance or gait disturbances, memory problems, painful tongue: these signs point toward vitamin B12 deficiency, more common in vegetarians, older adults, and those on certain medications. They warrant prompt medical evaluation. Our guide to vitamin B12 details these situations.
Raising your folate levels: diet, folic acid, or methylfolate
One thousand one hundred eight young Chinese women took folic acid at different doses for six months, then stopped. Plasma folates reached a plateau at three months, with an increase of 108% at 100 µg per day, 259% at 400 µg, and 460% at 4,000 µg. After stopping, they decreased rapidly.
Hao L, Yang QH, Li Z, et al. Am J Clin Nutr 2008;88(2):448-457. DOI : 10.1093/ajcn/88.2.448
The effect is dose-dependent and predictable: an analysis of 13 studies estimates that serum folates increase by 0.94 ng/mL for each additional 100 µg daily, twice as much between ages 40 and 65. For maintenance and prevention, the reference dose from trials is 400 µg per day; treatment doses for confirmed deficiency, which are higher and time-limited, are prescribed by the physician.
Folic acid or methylfolate? Both raise folates and lower homocysteine. Methylfolate does not need to be activated by the body, making it a logical option for carriers of the MTHFR variant, whose activation enzyme functions less efficiently; at equivalent dose, it raised red blood cell folates more in the 2006 trial, and equally in the 2026 trial. Folic acid remains the form used in most major trials, including those that demonstrated prevention of neural tube defects.
Diet subsequently maintains the results: legumes, leafy green vegetables, citrus fruits, eggs, daily, while limiting prolonged cooking in large amounts of water, which destroys some folates.
How long, and when to retest
The studies provide a clear timeline. Plasma folates reach a plateau around three months of daily supplementation, in both the 2008 and 2006 trials. Red blood cell folates continue to rise at 24 weeks and take six to twelve months to stabilize, the time it takes for red blood cells to renew. Homocysteine decreases by 20% at 0.4 mg per day.
In practice, retesting serum folates makes sense after approximately three months, and red blood cell folates after at least six months, following the WHO recommendation to reassess six to twelve months after an intervention. On the day of the test, it is better not to take your supplement just before the blood draw, to avoid measuring the last dose. And if the cause persists—poor diet, alcohol, medication—folates drop quickly when supplementation stops.
The plan, in order
All 8 B vitamins in active forms, including B9 as Quatrefolic® methylfolate and B12 as methylcobalamine, in one vegetable capsule per day, manufactured in France. To maintain your folate intake, in addition to a diet rich in vegetables and legumes. In case of confirmed deficiency, pregnancy, or treatment, the dose is determined with your physician., en une gélule végétale par jour, fabriquée en France. Pour entretenir ses apports en folates, en complément d'une alimentation riche en légumes et en légumineuses. En cas de carence avérée, de grossesse ou de traitement, la dose se décide avec votre médecin.
See B VitaminsOur guide to B vitamins covers what science proves.
Frequently Asked Questions
What are folates?
These are the natural forms of vitamin B9, found in foods, essential for DNA and red blood cell production. In the blood, the form that normally circulates is 5-methyltetrahydrofolate, or methylfolate. Folic acid is the synthetic form found in supplements and fortified foods.
What's the difference between folates and folic acid?
Folates are the natural forms of vitamin B9; folic acid is a stable synthetic form that the body must convert before using. At equivalent doses, its bioavailability is comparable to that of methylfolate. Both raise blood folates and lower homocysteine.
What is methylfolate?
It is 5-methyltetrahydrofolate, the active form of folates and the only form that normally circulates in the blood. In women of childbearing age, 416 µg of methylfolate raised red blood cell folates more than 400 µg of folic acid. In pregnant women, in glucosamine salt form, it raised red blood cell folates as much as folic acid.
Low serum B9 folates: what does my blood test mean?
Serum folates mainly reflect your intake over recent days. Below 7 nmol/L, or approximately 3 ng/mL, there is a risk of deficiency; below 10 nmol/L, approximately 4.4 ng/mL, deficiency is possible. Thresholds depend on the assay method: your laboratory's reference range is authoritative, and results should be interpreted alongside your blood count and vitamin B12.
Serum or red blood cell folates: which should be tested?
According to British hematology guidelines, serum folate testing is the first-line examination, with equivalent diagnostic value. Red blood cell folates reflect reserves from recent months; for neural tube defect prevention, the WHO recommends above 906 nmol/L, or 400 ng/mL, at the population level.
Low vitamin B9: what are the causes?
Low intake of vegetables, legumes and fruits, alcohol, malabsorption such as celiac disease, pregnancy, certain medications such as methotrexate, trimethoprim, anticonvulsants or sulfasalazine, and tobacco use. The MTHFR 677TT variant is associated with folates 13 to 16% lower than the most common form.
Should B12 be checked before taking folic acid?
Yes. At high doses, folic acid can correct the anemia from vitamin B12 deficiency while nerve damage progresses. Therefore, serum B12 is measured, supplemented if needed by methylmalonic acid testing. Tingling, balance or memory problems point to B12, not folates.
How long does it take to raise folate levels?
With daily supplementation, plasma folates reach a plateau around 3 months; red blood cell folates take 6 to 12 months to stabilize. A new serum folate test logically occurs after approximately 3 months, red blood cell folate testing after at least 6 months. After stopping, blood folates drop quickly.
Can you take too much vitamin B9?
EFSA sets the upper safety limit at 1,000 µg per day in adults for folic acid and methylfolate salts combined, due to the risk of masking B12 deficiency. In elderly people with low B12, very high serum folates have been associated with anemia and cognitive impairment.
Low vitamin B9 and pregnancy: what should I do?
Discuss this without delay with your doctor or midwife. Folate supplementation around conception reduces the risk of neural tube defects by 69% according to Cochrane review. During pregnancy, without supplementation, serum folates declined from 45.7 to 19.5 nmol/L between weeks 14 and 36 in a trial; folic acid prevented this decline.
Sources
The studies cited below were identified via PubMed.
- Pietrzik K, Bailey L, Shane B. Folic acid and L-5-methyltetrahydrofolate: comparison of clinical pharmacokinetics and pharmacodynamics. Clinical Pharmacokinetics, 2010;49(8):535-548. DOI
- Lamers Y, Prinz-Langenohl R, Brämswig S, Pietrzik K. Red blood cell folates increase more with [6S]-5-methyltetrahydrofolate than with folic acid in women of childbearing age. American Journal of Clinical Nutrition, 2006;84(1):156-161. DOI
- Ernawati E, Akyuni Q, Gumilar E, et al. Comparative bioavailability of 5-methyltetrahydrofolate and folic acid in pregnant women: randomized trial. Clinical Nutrition ESPEN, 2026;76:105107. DOI
- Chen MY, Rose CE, Qi YP, et al. Plasma folate levels corresponding to the erythrocyte folate threshold for optimal prevention of neural tube defects. American Journal of Clinical Nutrition, 2019;109(5):1452-1461. DOI
- Devalia V, Hamilton MS, Molloy AM. Recommendations for the diagnosis and treatment of cobalamin and folate disorders. British Journal of Haematology, 2014;166(4):496-513. DOI
- Monneret D, et al. Hemolysis indices and acceptable interference limits for biochemical tests and immunoassays. Scandinavian Journal of Clinical and Laboratory Investigation, 2015;75(2):162-169. DOI
- Pfeiffer CM, Sternberg MR, Hamner HC, et al. Inadequate thresholds lead to misinterpretation of folate status in the American population. American Journal of Clinical Nutrition, 2016;104(6):1607-1615. DOI
- Cordero AM, Crider KS, Rogers LM, Cannon MJ, Berry RJ. Optimal serum and erythrocyte folate concentrations in women of reproductive age for prevention of neural tube defects: WHO recommendations. Morbidity and Mortality Weekly Report, 2015;64(15):421-423. PubMed
- Okumura K, Tsukamoto H. Folates in smokers. Clinica Chimica Acta, 2011;412(7-8):521-526. DOI
- Medici V, Halsted CH. Folates, alcohol and liver disease. Molecular Nutrition and Food Research, 2013;57(4):596-606. DOI
- Roldan GA, et al. Causes of anemia and response to a gluten-free diet in untreated celiac patients: a two-year follow-up. American Journal of Gastroenterology, 2022;117(10):1684-1692. DOI
- McNulty B, McNulty H, Marshall B, et al. Continuing folic acid after the first trimester of pregnancy: a randomized trial in the second and third trimesters. American Journal of Clinical Nutrition, 2013;98(1):92-98. DOI
- Lambie DG, Johnson RH. Medications and folate metabolism. Drugs, 1985;30(2):145-155. DOI
- Linnebank M, et al. Antiepileptic drugs interact with serum folate and vitamin B12 levels. Annals of Neurology, 2011;69(2):352-359. DOI
- Halsted CH, et al. Sulfasalazine inhibits folate absorption in ulcerative colitis. New England Journal of Medicine, 1981;305(25):1513-1517. DOI
- Shea B, et al. Folic acid and folinic acid to reduce adverse effects of methotrexate in rheumatoid arthritis. Cochrane Database of Systematic Reviews, 2013;(5):CD000951. DOI
- Tsang BL, Devine OJ, Cordero AM, et al. MTHFR 677C>T polymorphism and blood folate levels: systematic review and meta-analysis. American Journal of Clinical Nutrition, 2015;101(6):1286-1294. DOI
- Green R, Datta Mitra A. Megaloblastic anemias: nutritional and other causes. Medical Clinics of North America, 2017;101(2):297-317. DOI
- Homocysteine Lowering Trialists' Collaboration. Dose-dependent effects of folic acid on blood homocysteine: meta-analysis of randomized trials. American Journal of Clinical Nutrition, 2005;82(4):806-812. DOI
- De-Regil LM, Peña-Rosas JP, Fernández-Gaxiola AC, Rayco-Solon P. Effects and safety of oral folate supplementation around conception to prevent birth defects. Cochrane Database of Systematic Reviews, 2015;(12):CD007950. DOI
- Miller JW, Smith A, Troen AM, et al. Excess folic acid and vitamin B12 deficiency: what are the clinical consequences? Food and Nutrition Bulletin, 2024;45(1 Suppl):S67-S72. DOI
- Morris MS, et al. Folate and vitamin B12 status, anemia, macrocytosis, and cognitive impairment in older Americans in the era of folic acid fortification. American Journal of Clinical Nutrition, 2007;85(1):193-200. DOI
- EFSA Scientific Committee on Nutrition, Turck D, et al. Scientific opinion on the upper safety limit for folates. EFSA Journal, 2023;21(11):e08353. DOI
- Hao L, Yang QH, Li Z, et al. Folate status and homocysteine according to folic acid dose and after its discontinuation: a randomized double-blind trial in young Chinese women. American Journal of Clinical Nutrition, 2008;88(2):448-457. DOI
- Wald NJ, Law MR, Morris JK, Wald DS. Quantifying the effect of folic acid. The Lancet, 2001;358(9298):2069-2073. DOI
