Comment on standardization for the assay values of serum folate as measured by automated analysis: Measurements using SRM 1955 as a calibration material

In cooperation with the Japan Committee for Vitamin Laboratory Standards, the Committee on Nutrition of the Japan Society of Clinical Chemistry has issued a statement on the standardization in Japan of serum folate measurement (sum of 5-methyltetrahydrofolic acid, 5-formyltetrahydrofolic acid, tetrahydrofolic acid, and folic acid). The repeatability and reproducibility-within-laboratory variations presented in the automated analysis (i.e., UniCel, Centaur and Elecsys) confound the diagnosis of hypovitaminosis folate deficiency, and required standardization from clinical and nutritional laboratories. Based on me study conducted by the World Health Organization (WHO), the National Institute of Standards and Technology (NIST) together with Centers for Disease Control and Prevention (CDC) developed a Standard Reference Material (SRM 1955). The SRM 1955 consisted of three levels, the assigned values of which were determined using a liquid chromatography/tandem mass spectrometry (i.e., the reference measurement procedure). The information concentrations of total folate were 2.6 ng/ mL for Level I, 5.8 ng/mL for Level II and 18.0 ng/mL for Level III. Using values of microbiological assay related to the assigned values of SRM 1955 as a comparison value, we investigated the possibility of standardization for the assay values of serum folate as measured by the automated analysis. When the observed values of SRM 1955 and 50 patient sera as measured by automated analysis were compared with the comparison values, the values by UniCel (for levels I and III) and Elecsys (for levels II and III) were plotted within a 95% prediction interval of regression line. The value by Centaur for Level II was plotted outside the 95% prediction interval, while the normalized residuals obtained from three levels of SRM 1955 by three automated analyses were all plotted within ±3.0 intervals. We concluded that SRM 1955 had a physicochemical characterization similar to that of native human serum. In the measurements of 50 patient sera by three automated analyses, we calibrated their observed values for the reference concentration values of SRM 1955. Based on the nutritional criteria classified as deficiency (<3.0 ng/mL), marginal deficiency (3.0-5.9 ng/mL) and low risk (≥ 6.0 ng/ mL), we compared the patients' nutritional status before and after correction with SRM 1955. By calibration for SRM 1955, both the number of patients with marginal deficiency misclassified as deficiency and patients with low risk misclassified as marginal deficiency were decreased. Although the increased number of patients with marginal deficiency misclassified as low risk needed to be further investigated, overall, the decreased number of misclassification verified the usefulness of calibration for SRM 1955. In this study, we used a method-specific calibration procedure for automated analysis, and the observed values were then calibrated for the assigned values of SRM 1955. In the future standardization, we expect all manufacturers to use SRM 1955 in assigning the values of their calibrator product for automated analysis.