Diffusion kinetics in blood during haemodialysis and in vivo clearance of inorganic phosphate

Contradictory data are reported in the literature concerning the diffusion kinetics of inorganic phosphates (iPh) between red blood cells and plasma d uring haemodialysis. Accordingly, we performed mass balance and equilibrati on studies to analyze the diffusion kinetics of iPh in vivo and in vitro. M ass balance analysis shows that iPh is only cleared from the plasma volume and thus that it practically does not diffuse from red blood cells to plasm a during the short time lapse of blood transit through the haemodialyzer. I n vitro equilibration studies of blood drawn at the filter outlet show that at room temperature there is a slow, limited, and almost linear net efflux of iPh during the 4 h that follow blood drawing. Our results point out: (1 ) that the in vivo clearance of iPh should be exclusively determined as pla sma clearance, and (2) that for accurate clearance determinations the iPh c oncentrations should be measured in blood samples centrifuged within at mos t Ih after blood drawing. Whole-blood clearance determinations - as well as the in vitro dialyzer data - largely overestimate (>30%) the real in vivo dialyzer performance. Copyright (C) 2001 S. Karger AG, Basel.