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.