Nonstandard abbreviations: NEQAS, National External Quality Assessment Service; FBP, folate binding protein; 5-methylTHF, 5-methyltetrahydrofolate acid; TFOL, total folate; ID-LC-MS/MS, isotope dilution-liquid chromatography-tandem mass spectrometry; PGA, pteroylglutamic acid; 5-formylTHF, 5-formyltetrahydrofolic acid; THF, tetrahydrofolic acid
; hmTHF, 4-[alpha]-hydroxy-5methylTHF; EQA, External Quality Assessment; MLTM, method laboratory trimmed mean; ALTM, all-laboratory trimmed mean; gCV, geometric CV; IS, international standard.
 Nonstandard abbreviations: LC-MS/MS, liquid chromatography-tandem mass spectrometry; BR, Bio-Rad radioassay; 5C[H.sub.3]THF, 5-methyltetrahydrofolic acid; THF, tetrahydrofolic acid
; TFOL, total folate; 5CHOTHF, 5-formyltetrahydrofolic acid; 5,10CH=THF, 5,10-methenyltetrahydrofolic acid; SSR, sum of squared residuals; PRESS, predicted residual sum of squares; FA, folic acid.
The low-QC pool contained 5CH3THF; 5-formyltetrahydrofolic acid (5CHOTHF); 5,10CH=THF; and tetrahydrofolic acid
(THF) at 33.9, 6.70, 25.5, and 68.2 t~,g/L of WB, respectively (likely MTHFR T/T genotype).
We used multiple-reaction monitoring (MRM) transitions for the following analytes (IS): 5C[H.sub.3]THF ([sup.13][C.sub.5]-5C[H.sub.3]THF), m/z 460 (465)[right arrow]m/z 313; 5CHOTHF ([sup.13][C.sub.5]-5CHOTHF), m/z 474 (479)[right arrow]m/z 327; FA ([sup.13][C.sub.5]-FA), m/z 442 (447)[right arrow]m/z 295; tetrahydrofolic acid
(THF; [sup.13][C.sub.5]-THF), m/z 446 (451)[right arrow]m/z 299; 5,10CH=THF ([sup.13][C.sub.5]-5C[H.sub.3]THF was used as pseudo-IS), m/z 456 [right arrow] m/z 412; and 10CHOFA ([sup.13][C.sub.5]-FA was used as pseudo-IS), m/z 470 [right arrow] m/z 295.