RBC folate optimal range and reference range

Red cell folate reflects folate status over the life of the red cell, roughly the previous three months. The band starts at 305 nmol/L (135 ng/mL), the deficiency cut-off for the microbiological assay used by the UK National Diet and Nutrition Survey. Immunoassays read on a different scale and some laboratories set their lower limit far higher (366 ng/mL at one large US laboratory), so read a result against the reporting laboratory's own interval first. For women who could become pregnant, population surveys use a separate, higher threshold of 748 nmol/L for neural tube defect risk; Sera does not score against it. A high result is not flagged: no laboratory or guideline defines an upper cut-off, and supplementation routinely carries red cell folate above the plotted range.

Folate measured inside your red blood cells rather than in the liquid part of the blood. Red cells live about 120 days and take up folate only while they are being made, so this reflects your folate status over months.

Serum folate can look normal a day after one folate-rich meal, which is why this version is ordered when the question is long-term status. One important catch: it reads falsely low when B12 is low, because B12 is needed to trap folate inside the cell. A low red cell folate alongside a low or borderline B12 points at the B12 first. A recent blood transfusion makes the result uninterpretable. Sera puts no upper bound on it: there is no high-folate pathology and no evidence-based ceiling to draw one at, so a high result on methylfolate supplementation is expected rather than a finding.