A membrane ultrafiltration procedure for determining diffusible calcium in serum.

USE OF HIGH-FLUX, preformed ultrafiltration membranes (Diaflo; Amicon Corp., Lexington, Mass.) for separating microand macromolecular constituents of biological fluids has been adequately documented (1-3). A class of these membranes has been formulated into a cone configuration (Centriflo; Amicon Corp.) amenable for use in a laboratory centrifuge. These membranes, highly retentive for the serum protein components, permit passage of low molecular-weight moieties and ionic species. Since isolation of the diffusible (nonproteinbound) calcium fraction of serum by more conventional ultrafiltration is laborious and must be done under rigorous control, the Centriflo membrane was evaluated for its applicability to this separation.