Differences in the distribution of urea and creatinine between red cells and plasma in normal and azotemic blood as assessed by autoanalyzer and manual chemical methods.

It has been proposed that urea and creatinine may bind to red cell constituents. In the present studies, whole blood and plasma concentrations of urea and creatinine were compared using autoanalyzer techniques (that require solutes to pass through a dialysis membrane) and manual techniques (with no dialysis membrane). Blood samples from 11 normal and 10 azotemic subjects were studied. Concentration differences between plasma water and red cell water for these solutes were significant in normals with standard manual but not with autoanalyzer methods. Accordingly, red cell water concentrations in normals were greater with manual methods than with autoanalyzer methods. In azotemics, differences between red cell and plasma water concentrations and differences in red cell water concentrations between methods were proportionately less. The findings suggest that accumulation of these solutes in the red cell in azotemia is predominantly in the freely diffusable form. Any chemical interference of red cell proteins and/or solute binding to red cell constituents yields significant discrepancies between manual and autoanalyzer methods only at lower BUN and creatinine concentrations.