Folate Blood Tests: Serum vs Red-Cell Folate and What Results Mean

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Tiredness, a sore tongue, pins and needles or memory problems can come from low folate, low B12 or something else entirely. NHS says to see a GP if you think you may have a B12 or folate deficiency, because some problems it causes can become irreversible if treatment is late. [1] If you are pregnant or planning pregnancy, ask your own care team about folate; official advice for pregnancy is covered in folic acid and folate before and during pregnancy. This page is information only.

The main blood tests

An earlier review series on biomarkers lists the same three, serum folate, RBC folate and plasma homocysteine, as the available folate biomarkers. [4] The sources stress different points. The British guideline says serum is enough as a first test, while the laboratory review notes that red-cell folate gives a longer-term picture. Which test is used is a clinician's choice.

What a result looks like

MedlinePlus gives a normal range of 2.7 to 17.0 ng/mL (6.12 to 38.52 nmol/L) for the folate blood test, and says ranges vary slightly among labs. [5] That is one US encyclopedia's example, not a universal standard. Use the range on your own report, and remember that units differ (ng/mL or nmol/L). MedlinePlus adds that lower-than-normal levels may reflect poor diet, malabsorption such as celiac disease, or malnutrition. [5]

Test it together with B12

Low folate and low B12 can cause the same blood picture. MedlinePlus describes megaloblastic anemia as a form of anemia that can be due to low folic acid or low B12, and says the folate test is often done with a B12 test. [5] The reason matters for treatment. NHS describes the two causes separately. For example, folate deficiency can come from diet, malabsorption, losses in dialysis or liver damage, and some medicines. [6] A high folate result should be followed by checking B12 status, according to the laboratory review. [2] See folate deficiency and megaloblastic anemia.

What can skew the result

• Fasting: MedlinePlus says not to eat or drink for 6 hours before the test. [5]
• Supplements: your provider may tell you to stop certain medicines, including folic acid supplements, that could interfere. [5] Do not do this yourself. Ask first.
• Medicines: alcohol, birth control pills, estrogens, methotrexate, penicillin and some seizure medicines can lower measured folic acid. [5]
• Inflammation and illness: a 2023 review says inflammation lowers plasma concentrations of folic acid among other micronutrients, so a marker like C-reactive protein is needed. [7]
• Symptoms and results can disagree. The laboratory review says that when results and strong clinical features of deficiency disagree, treatment should not be delayed. [2]

Folate, folic acid and MTHFR

Folate, folic acid and methylfolate are different forms and are explained in folate vs folic acid vs methylfolate. Gene-variant testing (MTHFR) does not replace a folate blood test; see MTHFR variants and folate.