In addition to the data from the 2 distributions, we had TFOL data from the CDC IDLC-MS/MS method on routine EQA samples distributed between June 2008 and October 2009.
Linear regression analysis was performed using the ID-LC-MS/MS mean values for TFOL converted to scheme units ([micro]g/L) as the nonvariable (x) paired with all the all-laboratory data or method data as the variable (y).
All 3 pools analyzed by the ID-LC-MS/MS reference method gave calculated TFOL values within the analytical linear assay ranges quoted by the manufactures for the FBP assays.
The LC-MS/MS method results showed that the samples circulated in the survey contained 5-methylTHF, 5-formylTHF, and PGA in concentrations that might be expected in normal healthy subjects in populations not receiving staple foods fortified with PGA, and TFOL was well below the 22.
This IS has values for TFOL assigned by 2 reference methods for serum folate (21), including the method used in this study.
The tendency for the FBP assays to overrecover PGA suggests that these assays may be increasingly inaccurate when measuring TFOL in samples that may contain increased levels of PGA, and their use would be more appropriate to the measurement of sera with TFOL <22.
Interestingly, the TFOL concentration by either LC-MS/MS or microbiologic assay was not affected by the genotype in either sample set.
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.
We found similar good agreement for TFOL between our LC-MS/MS and MA methods (level 1, 6.
The BR was initially marketed (QuantaPhase I) with calibrator concentrations to match MA performance, but questions raised by Levine (8) prompted introduction of the QuantaPhase II assay in 1993 with spectrophotometrically verified FA calibrator concentrations, resulting in a 30% downward shift in TFOL concentrations.
Some advantages of the LC-MS/MS compared with the MA are that it provides information on the different folate species in addition to TFOL and it is less prone to interferences such as antibiotics.
1) Nonstandard abbreviations: FA, folic acid; 5MeTHF, 5-methyltetrahydrofolic acid; THF, tetrahydrofolic acid; IS, internal standard; LC/MS, liquid chromatography-mass spectrometry; SPE, solid-phase extraction; MS/MS, tandem MS; 5FoTHF, 5-formyltetrahydrofolic acid; PteGlu, pteroylmonoglutamic acid; MRM, multiple-reaction monitoring; QC, quality control; LOQ, limit of quantification; LOD, limit of detection; and TFOL
, total folate.