Parameterization of red blood cell elongation index--shear stress curves obtained by ektacytometry.

Measurement of red blood cell (RBC) deformability by ektacytometry yields a set of elongation indexes (EI) measured at various shear stresses (SS) presented as SS-EI curves, or tabulated data. These are useful for detailed analysis, but may not be appropriate when a simple comparison of a global parameter between groups is required. Based on the characteristic shape of SS-EI curves, two approaches have been proposed to calculate the maximal RBC elongation index (EI(max)) and the shear stress required for one-half of this maximal deformation (SS(1/2)): (i) linear Lineweaver-Burke (LB) model; (ii) Streekstra-Bronkhorst (SB) model. Both approaches have specific assumptions and thus may be subject to the measurement conditions. Using RBC treated with various concentrations of glutaraldehyde (GA) and data obtained by ektacytometry, the two approaches have been compared for nine different ranges of SS between 0.6-75 Pa. Our results indicate that: (i) the sensitivity of both models can be affected by the SS range and limits employed; (ii) over the entire range of SS-data, a non-linear curve fitting approach to the LB model gave more consistent results than a linear approach; (iii) the LB method is better for detecting SS(1/2) differences between RBC treated with 0.001-0.005% glutaraldehyde (GA) and for a 40% mixture of rigid cells but is equally sensitive to SB for 10% rigid cells; and (iv) the LB and SB methods for EI(max) are equivalent for 0.001% and 0.003% GA and 40% rigid, with the SB better for 0.005% GA and the LB better for 10% rigid.