Limited Dynamic Range of a New Assay for Serum Folate

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 calibrators 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 Pharmaceuticals Inc.) and the Quantaphase II (Bio-Rad Laboratories). Both of these are competitive RIAs.We selected 100 patient samples from among serum specimens received in the laboratory for determination of serum folate to obtain a concentration range up to 68 nmol/L (30 μg/L) as measured by the SimulTrac-SNB assay. The sera were frozen at −20 °C until analysis. After thawing, all three …