Folate (folic acid) blood test: what it measures, normal levels, and what low folate levels mean

Folate is a B vitamin (B9) found in leafy greens, legumes, liver and — since 1998 in the United States — in fortified flour, bread, cereal and rice as folic acid. Inside cells it carries single-carbon units for building DNA and for converting homocysteine back to methionine. The serum folate test measures the vitamin circulating in the blood, which reflects recent intake over the preceding days to weeks; a red blood cell (RBC) folate test measures the amount stored inside red cells, which reflects status over the previous two to four months and is less swayed by a recent meal or supplement. Most laboratories and clinicians use serum folate first, with RBC folate reserved for ambiguous cases.

Folate and vitamin B12 work in the same biochemical pathway, and a deficiency of either produces the same blood picture: large red cells (high MCV), a macrocytic anemia, and a raised homocysteine. That is why the two are nearly always tested together, and why giving folic acid to someone whose real problem is B12 deficiency can improve the anemia while the nerve damage of B12 deficiency continues — the single most important caution in this area.

Folate stores last only a few months, so deficiency shows faster than B12 deficiency. The early symptoms are those of a slowly developing (megaloblastic) anemia:

Unlike B12 deficiency, folate deficiency does not damage nerves — so tingling, numbness or balance problems point toward B12, not folate — and the two are tested together because treating low folate alone can mask a B12 problem. In pregnancy, low folate raises the risk of neural tube defects, which is why supplementation starts before conception.

A high folate almost always means supplements or fortified foods and has no known direct harm, though a very high folate alongside a low B12 is a combination clinicians look for.

Representative adult reference ranges for serum folate run from about 3.4 to 25 ng/mL (some laboratories report up to 20 or above 40; the upper limit matters little). Values below about 3 ng/mL indicate deficiency; 3–5 ng/mL is a borderline zone where a raised homocysteine or RBC folate clarifies the picture. RBC folate reference ranges are much higher — roughly 140–628 ng/mL — and are not interchangeable with serum values. Because fortification raised population levels substantially, 'high' folate is common and, on its own, meaningless: it reflects supplements or a fortified diet.

Serum folate can be misleading in two directions. A single folate-rich meal or a multivitamin taken that morning raises it transiently, so a borderline result is repeated fasting; and a recent change in diet can normalize serum folate within days while tissue stores remain low. Sample handling matters too — hemolysis releases red-cell folate into the serum and falsely raises the result.

Because fortified grains supply most Americans with adequate folic acid, deficiency today usually has a specific cause. From most to least common:

The blood shows it first: red cells enlarge (MCV rises above about 100 fL), hemoglobin falls, and the smear shows oval macrocytes and hypersegmented neutrophils; white cells and platelets can fall as well. Symptoms are those of anemia — fatigue, breathlessness on exertion, pale skin, palpitations — plus a sore, smooth tongue, mouth ulcers, diarrhea and irritability. Unlike B12 deficiency, folate deficiency does not cause nerve damage, numbness or balance problems; the presence of those symptoms points to B12 regardless of the folate result. Low folate also raises homocysteine, which is associated with cardiovascular risk, though lowering it with supplements has not reduced heart attacks in trials.

The consequence that drives public-health policy is in pregnancy: low maternal folate in the weeks around conception raises the risk of neural tube defects (spina bifida, anencephaly). Because the neural tube closes before most pregnancies are recognized, the recommendation is that everyone who could become pregnant take 400 micrograms of folic acid daily — the reason grains are fortified — with higher doses advised for those who have had an affected pregnancy or take certain anticonvulsants.

Low folate and low B12 look the same on a blood count, so a macrocytic anemia is always worked up with both vitamins. Homocysteine rises in both deficiencies; methylmalonic acid (MMA) rises only in B12 deficiency, which makes MMA the test that separates them when B12 is borderline. A normal folate with a high MCV points toward B12, alcohol, liver disease, hypothyroidism, some medications or a marrow disorder. A low folate with a normal B12 and MMA is a folate problem. And a high folate — common with supplements — is a reason to be sure B12 is adequate, because folic acid can correct the anemia of B12 deficiency and let the neurological damage progress unnoticed.

Confirm it was a fasting sample without a supplement that morning, then look for the cause with a clinician: alcohol intake, diet, gut symptoms suggesting malabsorption, and the medication list. Check vitamin B12 (and MMA if borderline) before any folic acid is started, and a CBC to see whether the deficiency has produced anemia. Treatment is straightforward — oral folic acid replaces stores within weeks — but the cause determines whether it is a short course or ongoing, and whether celiac testing, a review of alcohol use or a medication change is needed. Recheck the CBC after a few weeks; the reticulocyte count rises within days of starting treatment and hemoglobin recovers over one to two months. Anyone planning pregnancy should be on 400 micrograms of folic acid daily regardless of their test result.

Serum folate is a routine test at Quest and Labcorp, available through a clinician or directly through TestWell with physician review, with results in a few business days. Draw it fasting, and skip any multivitamin or B-complex supplement the morning of the test, since a dose taken hours earlier raises the level. It is almost always ordered with vitamin B12 — and with a CBC when anemia or a high MCV is the question — because the two deficiencies look identical and are treated differently; the TestWell B12 and folate combination covers both from one draw. If a result is borderline, RBC folate or homocysteine can be added on a repeat.