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.
Fazili
Zia
Z
Division of Laboratory Sciences, National Center for Environmental Health, Centers for Disease Control and Prevention, Atlanta, Georgia 30345, USA.
Pfeiffer
Christine M
CM
Zhang
Mindy
M
D001431:Bacteriological Techniques; D002853:Chromatography, Liquid; D005492:Folic Acid / Q000097:blood*; D006801:Humans; D011868:Radioisotopes; D011933:Reagent Kits, Diagnostic; D044967:Serum; D053719:Tandem Mass Spectrometry
D011868:Radioisotopes; D011933:Reagent Kits, Diagnostic; D005492:Folic Acid