[3] 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.