Reference Interval for Korean Serum Folate Assay Traceable to the WHO International Standard

BACKGROUND Little is known about the reference interval of serum folate concentration if using recently re-standardized assays traceable to the World Health Organization (WHO) international standard reference 03/178 in a Korean population. This study aimed to investigate serum folate levels in Korean subjects without macrocytic anemia or increased homocysteine, for the assessment of folate deficiency. METHODS We retrospectively reviewed data from Korean adults whose hemoglobin, mean corpuscular volume, and serum total homocysteine values were within reference limits. RESULTS The median (interquartile range) serum folate level was 7.8 (5.4 - 12.6) ng/mL in men and 10.2 (6.9 - 15.6) ng/mL in women. The reference interval for serum folate (2.5th and 97.5th percentiles) ranged from 2.9 to 38.0 ng/ mL. From among 723 Korean adults, the lower limit of reference intervals of serum folate for folate deficiency, defined as the 2.5th percentile, was 2.9 ng/mL. The prevalence of folate deficiency was higher in men (6.5%) than in women (1.2%, p < 0.05) when a cutoff value of 3.0 ng/mL was applied. Using the cutoff value of 4 ng/mL for folate deficiency, which is in accordance with the instructions from the manufacturer of the new assay and the WHO 2012 guideline for homocysteine as a metabolic indicator before assay standardization, about 5% of subjects were reclassified as folate deficient. CONCLUSIONS Our study suggests that any change of reference limits using a re-standardized assay needs to be verified in clinical laboratories.