Folate (Serum)

What is Folate (Serum)?

Serum folate measures the vitamin B9 circulating in your blood, reflecting mostly what you've eaten in the past few days to weeks.

Folate is vitamin B9, the nutrient your body uses to build DNA, make new cells, and run the one-carbon metabolism cycle that also involves vitamin B12 and homocysteine (the vitamin B12 page covers how your body absorbs and stores B12; this page focuses on folate itself). A serum folate test measures how much of the vitamin is dissolved in the liquid part of your blood at the moment of the draw, which mostly tracks what you've eaten or supplemented over the past several days to a few weeks, according to the College of American Pathologists. That's different from an RBC (red blood cell) folate test, which measures folate trapped inside red blood cells when they were made in your bone marrow. Because a red blood cell lives about three months, RBC folate reflects your folate status over roughly that whole window instead of just recent days. That sounds like it should make RBC folate the better test, and clinicians used to think so, but CAP's 2023 clinician guidance says the two tests give equivalent diagnostic information in practice, RBC folate has more analytical variability due to extra lab processing steps, and routine testing of both is discouraged. Serum folate is now the test ordered first and often the only one needed, with RBC folate reserved for select cases such as suspected hereditary folate transport problems. The other piece of folate's story is fortification. The United States and Canada were the first countries to mandate folic acid fortification of enriched grain products, starting in 1998, a decision that followed research linking folate to prevention of neural tube birth defects. The NIH's Office of Dietary Supplements credits mandatory fortification with a roughly 28% drop in neural tube defect rates in the US. As of a 2025 global review, 69 countries had adopted mandatory fortification, while most of the European Union relies on voluntary fortification instead, a distinction that shows up in the numbers: one comparison found about 4 neural tube defects per 10,000 births in mandatory-fortification countries versus roughly 8 in voluntary-fortification countries and 10 where there's no fortification at all. Frank folate deficiency, once a meaningful cause of anemia in the US, is now uncommon largely because of this policy. The clinically important wrinkle is what happens when folate is abundant but B12 isn't. High folate can correct the enlarged red blood cells that usually flag a B12 deficiency, without addressing the B12 shortage itself. That means the blood test result that would normally prompt someone to get checked for B12 problems can look normal, while the nerve damage B12 deficiency causes keeps progressing unseen. This is sometimes called the folate trap, and it's the main reason doctors often check folate and B12 together rather than folate alone.

Typical reference context

Labs vary somewhat in their own cutoffs and units, so always read a result against the reference range printed on that specific report, not a number from a different lab or a different country's guidelines.

What changed recently

A September 2026 systematic review in Nutrients (Frye and Rossignol) took a fresh look at unmetabolized folic acid, or UMFA, the form of folic acid that shows up in blood when intake outpaces the body's ability to convert it. The review's headline finding: current evidence does not establish that UMFA independently causes harm. It found switching from folic acid to the natural folate form (6S)-5-MTHF appears to lower circulating UMFA, but didn't produce meaningfully different results on standard markers like red blood cell folate or homocysteine, and existing studies on cancer recurrence and child neurodevelopment linked to UMFA were described as mostly null or protective. The authors were also clear that the evidence base is thin: only two or three trials exist per outcome, none designed to formally prove the folate forms are equivalent, so the debate isn't settled so much as still short on data.

Tests often checked alongside this

Folate is rarely ordered completely alone. A complete blood count (CBC) shows whether red blood cells are enlarged, a pattern folate deficiency and B12 deficiency both produce. Vitamin B12 (Cobalamin) is checked alongside folate specifically because of the masking issue described above. Some clinicians also add Homocysteine or Methylmalonic Acid (MMA) when the picture is unclear, since MMA rises specifically with B12 deficiency but not folate deficiency, helping tell the two apart.

What to ask your doctor

• Did you order serum folate, RBC folate, or both, and why that choice for me?
• If my folate is high and B12 is borderline, how do we make sure B12 deficiency isn't hiding behind it?
• Should I hold off on my multivitamin or folic acid supplement before this blood draw, and for how long?

This glossary entry is for informational purposes only and is not a substitute for professional medical advice. Read our Terms of Use.

Frequently asked questions