Data reduction methods for ektacytometry in clinical hemorheology

Laser-diffraction ektacytometry is a generally accepted technique for measuring RBC deformability induced by fluid shear stress (SS) and yields paired elongation index-SS data at several levels of stress. Unfortunately, comparison of results is hindered by the lack of simple indices that accurately characterize these data. Several mathematical models have been proposed, including those developed for analysis of enzyme kinetics (Lineweaver-Burk, Eadie-Hofstee) and curve fitting (Streekstra-Bronkhorst). All of these analytical approaches provide a value for cell deformation at infinite stress (EI_max) and the shear stress required to achieve one-half of this deformation (SS_1/2); the use of non-linear regression is essential when calculating these parameters. While the current models provide equivalent results for normal RBC if used with non-linear regression, EI_max and SS_1/2 are not always concordant for cells with abnormal mechanical behavior. This technical note examines such differences for three conditions: glutaraldehyde treatment, mechanical stress and non-isotonic media. It was found that none of the models yield completely satisfactory values for EI_max and SS_1/2, especially if there are large changes of EI_max. However, the ratio of SS_1/2 to EI_max (SS_1/2/EI_max) is much less affected by these problems, has similar power (i.e., standardized difference) as SS_1/2 and EI_max and is more robust in reflecting alterations of deformability. We thus conclude that the SS_1/2/EI_max ratio can be used when reporting and comparing various populations of RBC or cells obtained from subjects having different clinical states.