Limited dynamic range of a new assay for serum folate.

To the Editor: Recently, several new nonisotopic protein-binding folate assays have become available. We have studied the AxSYM folate assay (Abbott Laboratories , Abbott Park, IL), in which folate is quantified by measurement of unoccupied binding sites on the folate-binding protein (1). The cali-brators for the assay cover the range 0 – 45 nmol/L. In the absence of an internationally accepted reference method or material (2), we have compared the performance of the AxSYM method with two methods commonly used nationally and internationally, namely the SimulTrac-SNB (ICN Fig. 1. Comparison of results for 100 serum folate samples. (A), AxSYM folate assay vs Quantaphase II folate assay (y ϭ 0.63x ϩ 2.8 nmol/L; r ϭ 0.824; n ϭ 100). (B), difference of AxSYM and Quantaphase II folate plotted against the average of AxSYM and Quantaphase II folate; horizontal lines are the mean bias (⅐ ⅐ ⅐ ⅐ ⅐) and Ϯ 2 SD values (Ϫ ⅐ Ϫ ⅐ Ϫ ⅐ Ϫ). (C), AxSYM folate assay vs SimulTrac folate assay (y ϭ 0.58x ϩ 2.7 nmol/L; r ϭ 0.823; n ϭ 100). (D), difference of AxSYM and SimulTrac folate plotted against the average of AxSYM and SimulTrac folate; 1, patients with renal failure. (E), linearity study; (——), least-squares linear regression line for the points at 10 –30% high sample.