Comparison of serum folate species analyzed by LC-MS/MS with total folate measured by microbiologic assay and Bio-Rad radioassay.

BACKGROUND The Bio-Rad QuantaPhase II radioassay (BR), used for 25 years to measure total folate (TFOL) concentrations for the National Health and Nutrition Examination Survey (NHANES), will be discontinued in 2007. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) or a microbiologic assay (MA) will be used in the future. METHODS We measured folate species by LC-MS/MS and TFOL by MA and BR in 327 serum samples. RESULTS LC-MS/MS measured 5-methyltetrahydrofolic acid (5CH(3)THF; 82%), folic acid (FA; 8%), 5-formyltetrahydrofolic acid (5CHOTHF; 6%), tetrahydrofolic acid (THF; 4%), and 5,10-methenyltetrahydrofolic acid (5,10CH=THF; 0%). The sum of the folate species correlated well with TFOL measured by MA (R(2) = 0.97) and BR (R(2) = 0.91). Compared with LC-MS/MS results, MA and BR values were significantly lower (-6% and -29%, respectively); however, these differences were concentration dependent. The MA almost completely recovered folates added to serum samples except for FA [69% (3%)] and THF [36% (10%)]. The BR underrecovered 5CH(3)THF [61% (9%)] and 5CHOTHF [38% (14%)] and overrecovered 5,10CH=THF [234% (32%)]. Multiple linear regression models with log-transformed data yielded a good fit for converting BR data to MA or LC-MS/MS data and MA data to LC-MS/MS data. CONCLUSIONS The good correspondence between the sum of folate species determined by LC-MS/MS and TFOL determined by MA makes these 2 assays interchangeable. The BR produces much lower results, on average, probably because of 5CH(3)THF underrecovery. The conversion equations provided could be used for future NHANES time trend analyses.