Folate

Folate is a B vitamin (B9) essential for DNA synthesis and for making healthy red blood cells. Because dividing cells depend on it, a shortage first shows up in tissues that turn over quickly, like the bone marrow. It's frequently tested alongside vitamin B12 because the two work together and their deficiencies overlap.

For serum folate, a common reference is roughly above 4 ng/mL, with lab ranges often running from about 2.7 to 17 ng/mL. Values below the low end suggest deficiency, while high values usually just reflect supplementation or a folate-rich diet and aren't a concern. As always, interpret the number against your specific lab's range.

Serum folate reflects recent intake and can swing quickly with a few good or bad meals, so it's a short-term snapshot. Red blood cell (RBC) folate is incorporated into cells as they form and reflects folate status over the prior few months, making it a steadier measure of true stores. When serum folate is borderline or diet has recently changed, some clinicians order RBC folate for a more reliable picture.

Both vitamins are needed to build red blood cells, and a deficiency in either can cause the same picture — large, immature red cells known as macrocytic anemia (often flagged by a high MCV). Because they look similar on a blood count, doctors check both to find which one is short. This matters because correcting folate alone while B12 is low can mask an ongoing B12 problem, so they're evaluated as a pair.

The usual causes are inadequate dietary intake, heavy alcohol use, and certain medications that interfere with folate metabolism. Conditions that impair absorption in the gut can contribute as well. Because low folate often travels with other nutritional gaps, a low result is usually looked at alongside B12 and the rest of your blood count rather than in isolation.