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Issues in immunoassay standardization: the ARCHITECT Folate model for intermethod harmonization.

Extensive variability among folate assay methods has been well described. A FLAIR round-robin study of serum and whole-blood folate results across 11 laboratories in seven European countries reported overall CVs of 18–41% (1). A CDC round-robin survey of 20 laboratories reported two- to ninefold differences in folate values among methods and laboratories (2). With increasing awareness of the nutritional importance of folate, both studies concluded that there was an acute need for improved standardization of folate testing. Recently, a comparison of five automated serum and whole-blood folate assays was reported (3). In that study, the methods were compared with the Bio-Rad Quantaphase II radioassay. Between the methods, differences as high as 40% for serum folate and 250% for whole-blood folate were noted. The authors made reference to differences in standardization as likely contributors to the variation in serum folate results, and/or differences in preparation of the whole-blood hemolysates as contributing to the variability in whole-blood folate results. The report served to underscore that little has changed with respect to between-method variability in folate testing since the round-robin surveys 8–10 years ago.

Heterogeneous assays for folate measurement represent an interesting and challenging problem from a standardization standpoint. Technically, folate methods are not immunoassays because they all rely on a high-affinity folate-binding protein (FBP) as the specific capture entity rather than anti-folate antibodies.1 In addition, folate (as pteroylglutamic acid used for food fortification) is available as a stable, highly purified powder, which would seem to be ideal for gravimetric preparation of standards. With both analyte and analyte binders seemingly normalized across methods, why the large differences?

The answer, which we discuss later, highlights once again the general difficulty in standardizing immuno- and bioreceptor-based assays in an absolute sense. Stenman (4) recently wrote a thoughtful review of the scientific and practical difficulties …