FLAIR intercomparisons on serum and red cell folate.

Interlaboratory variation in the results for serum and red cell folate was assessed as part of the EC FLAIR Concerted Action No 10 with 11 participating laboratories from 7 European countries. In the first intercomparison freeze dried quality control serum samples at two different levels were analysed using different assay methodologies (microbiological, radioassay, HPLC and chemilumiscence). Considerable variability was observed both between different methods ("over-all" coefficient of variation (CV) 18-41%), as well as between laboratories using a similar assay kit/protocol. In the second intercomparison recovery studies were performed with sera spiked with calibrated standard preparations of pteroylmonoglutamic acid (PGA) or 5-methyltetrahydrofolic acid (5-MTHF). With radioassays average PGA recoveries ranged between 86-140% (mean 110%), for 5-MTHF between 99-144% (mean: 130%). In the third intercomparison with commercial whole blood control samples variabilities (CV's) between 36 and 63% were found. These results indicate that especially radioassays tend to overestimate serum folate content due to improper standard calibration. Also small differences in assay pH as well as matrix effects may contribute to the variability. These data stress the importance of improved standardization of (commercial) diagnostic kits and the provision of suitable reference materials with assigned folate levels spanning its physiological concentration range.