Quantitation of whole-blood total folate within defined MTHFR C677T genotype groups by isotope dilution-liquid chromatography-tandem mass spectrometry differs from microbiologic assay

Harmonizing the Calibrator and Microorganism Used in the Folate Microbiological Assay Increases the Comparability of Serum and Whole-Blood Folate Results in a CDC Round-Robin Study

Background

Harmonizing critical reagents for the folate microbiological assay (MBA) may improve among-laboratory comparability.

Objective

We assessed the comparability of the MBA for serum folate (S-FOL) and whole-blood folate (WB-FOL) in an international comparison study.

Methods

Eight laboratories obtained a kit containing CDC microorganism inoculum (chloramphenicol-resistant Lactobacillus rhamnosus), CDC calibrator (5-methyltetrahydrofolate), and 23 serum and WB hemolysate samples each. Laboratories analyzed the samples in single measurement over 2 d using 4 conditions: in-house microorganism and in-house calibrator (IH-MO & IH-CAL), in-house microorganism and CDC calibrator (IH-MO & CDC-CAL), CDC microorganism and in-house calibrator (CDC-MO & IH-CAL), and CDC microorganism and CDC calibrator (CDC-MO & CDC-CAL). We calculated geometric mean concentrations for each laboratory and condition and compared data to the CDC MBA (target).

Results

The among-laboratory arithmetic mean S-FOL concentrations for the 4 conditions were 30.2, 28.1, 30.0, and 29.9 (group 1, 5-methyltetrahydrofolate IH-CAL) compared with 35.3, 33.3, 33.6, and 30.7 nmol/L (group 2, folic acid IH-CAL), respectively; and 428, 405, 398, and 393 (group 1) compared with 469, 423, 477, and 418 nmol/L (group 2), respectively, for WB-FOL. Differences to the CDC MBA target values were smaller for group 1 (range across conditions; S-FOL: 9.9-21%; WB-FOL: 9.0-18%) compared with group 2 laboratories (S-FOL: 13-30%; WB-FOL: 16-32%) and smaller when CDC reagents were used compared with in-house reagents (S-FOL: 12% compared with 22%; WB-FOL: 13% compared with 25%). A linear mixed model estimated a small microorganism effect (S-FOL: 2.3%; WB-FOL: 2.3%) and a larger mean calibrator effect; folic acid compared with 5-methyltetrahydrofolate calibrator produced 12% higher S-FOL and 15% higher WB-FOL results. When laboratories used CDC reagents, the estimated among-laboratory variability was ∼10% for S-FOL and WB-FOL.

Conclusion

Harmonizing the calibrator and microorganism for the folate MBA has the potential to improve the among-laboratory comparability in future surveys.

Folate determination in human health: UPLC-MS/MS is the emerging methodology of choice

This Perspective provides a brief description of the essential role that folates play in human health, together with an overview of the various analytical methods that have been used for quantitation of folates in human populations over the past few decades. Essentially, folate methodology has evolved from microbiological assay-based, to binding-based technology and, more recently, to separation-based methodology. Separation-based methods initially used traditional LC in conjunction with various detection techniques, with the most recent methods utilizing UPLC-MS/MS. Current UPLC methods offer exceptional speed, sensitivity and quantitation ability for the monoglutamate folate isoforms. It appears that the only limitation to properly quantifying all folates as polyglutamates, some 40-50 species, is the current lack of corresponding stable-isotope standards. Clearly, UPLC-MS/MS is emerging as the 'method of choice' for the determination of folates, whether in support of basic research, clinical investigations or population studies.