Folate (Serum & Red Cell) Blood Test Explained
Serum folate reflects recent intake and fluctuates; red-cell folate reflects tissue stores. Learn which to order, cut-offs, and when to co-test B12.
Serum folate reflects recent intake and fluctuates day-to-day; red-cell folate is more stable but expensive. Serum folate < 7 nmol/L is deficient in most labs. ALWAYS check B12 before treating folate deficiency — folate replacement can mask B12 deficiency and precipitate neurological damage.
Folate is essential for DNA synthesis, red-cell production, and neural tube development. Deficiency causes megaloblastic anaemia similar to B12 deficiency but without neurological signs. Most common causes: poor diet (elderly, alcohol dependence), malabsorption (coeliac disease, IBD), increased demand (pregnancy, haemolysis), drugs (methotrexate, phenytoin, sulfasalazine). Serum folate is the standard first-line test; red-cell folate is preferred when serum is borderline or recently supplemented. Homocysteine rises in folate deficiency and normalises with treatment. Critical rule: check and correct B12 before starting folate — otherwise subacute combined degeneration of the cord may progress silently. UK guidance recommends 400 mcg folic acid daily preconception through 12 weeks gestation to prevent neural tube defects; 5 mg for high-risk (previous NTD, diabetes, obesity, antiepileptics).
- Serum folate deficient
- < 7 nmol/L (< 3 ng/mL)
- Red-cell folate deficient
- < 305 nmol/L
- Preconception dose
- 400 mcg/day folic acid
- High-risk pregnancy dose
- 5 mg/day
- Warning
- Always check B12 before treating folate
Serum vs red-cell folate
| Feature | Serum folate | Red-cell folate |
|---|---|---|
| Reflects | Recent intake (days) | Tissue stores (weeks–months) |
| Diet fluctuation | High | Low |
| First-line | Yes | No |
| Cost | Low | Higher |
| Best use | Screening | Confirming borderline/supplemented cases |
Causes of folate deficiency
Dietary: poor intake, chronic alcohol use, "tea and toast" elderly.
Malabsorption: coeliac disease, IBD, jejunal resection.
Increased demand: pregnancy, lactation, chronic haemolysis, exfoliative skin disease.
Drugs: methotrexate, trimethoprim, phenytoin, sulfasalazine.
- Macrocytic anaemia (MCV > 100 fL) + low folate — treat after excluding B12 deficiency.
- Pregnancy planning + previous neural tube defect — 5 mg folic acid daily.
- Folate deficiency + neurological symptoms — B12 must be co-tested urgently.
Low folate workup
- 1Serum folate < 7 nmol/L?Confirm B12 normal. Investigate diet, alcohol, malabsorption. Start folic acid 5 mg/day × 4 months.
- 2B12 also low?Treat B12 first (or concurrently) — never folate alone.
- 3Persistently low despite replacement?Test for coeliac disease (tTG-IgA).
Related questions people ask
Frequently asked questions
- Serum folate is first-line; red-cell folate confirms borderline.
- Always check B12 before folate treatment.
- Anaemia is macrocytic (MCV > 100 fL).
- Preconception folic acid prevents neural tube defects.
- Methotrexate, trimethoprim, and phenytoin are common culprits.
References
2 sources- BSHGuidelines for the diagnosis and treatment of cobalamin and folate disorders
British Society for Haematology.
onlinelibrary.wiley.com
- NHSFolic acid in pregnancy
UK guidance.
nhs.uk
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