Kinetic analysis of icodextrin interference with serum amylase assays

Patients treated with Extraneal peritoneal dialysis solution (Baxter Healthcare Corporation, Deerfield, IL, U.S.A.) have a significant decrease in serum amylase activity. The decline is reported to be due to interference of icodextrin in a routinely used laboratory assay. The present study was designed to investigate the kinetics of icodextrin interference in the amylase activity assay and to determine whether assay interference can account for the total decline in amylase activity. Plasma obtained from healthy volunteers was spiked with 0, 0.21, 0.71, and 3.6 mg/mL icodextrin. Amylase activity was determined using Sigma kit 577-10 (Sigma Diagnostics, St. Louis, MO, U.S.A.). Amylase activity in plasma samples spiked with 3.6 mg/mL icodextrin was also monitored while varying the concentration of the substrate (ET-G7-PNP) from the assay kit. Amylase activity decreased with increasing amounts of icodextrin and decreasing amounts of assay substrate. A 72.6% decrease in amylase activity was observed in samples spiked with 3.6 mg/mL icodextrin as compared with samples without icodextrin at a substrate level similar to that in the assay kit (0.71 mmol/L). Double reciprocal and Dixon plots indicate competitive inhibition of amylase activity by icodextrin. Icodextrin functions as a competitive inhibitor in the assay for amylase activity, as predicted by the structural similarities between icodextrin and the amylase assay substrate. The degree of icodextrin interference suggests that the entire decline in amylase activity observed in patients using Extraneal can be accounted for qualitatively by icodextrin interference. The amylase activity decline in patients treated with Extraneal is an artifact attributable to assay interference.