Osmotic error in erythrocyte volume determinations

Because of osmotic effects erythrocytes suspended in their native plasma do not have the same volume as the same erythrocytes suspended in Isoton. The discrepancy varies depending upon the osmolality and composition of the native plasma and the length of time the cells have been suspended in Isoton. consequently, the MCV recorded in an electronic particle counter (Coulter in this case) may differ markedly from the true in vivo MCV. A similar error affects the Coulter hematocrit, which is calculated from the MCV and the erythrocyte count. This matrix effect should be taken into account in any laboratory quality assurance program.