Detection of red cell aggregation by low shear rate viscometry in whole blood with elevated plasma viscosity

The viscosity of whole blood measured at low shear rates is determined part ly by shear resistance of the red cell aggregates present, stronger aggrega tion increasing the viscosity in the absence of other changes. Effects of c ell deformability can confound interpretation and comparison in terms of ag gregation, however, particularly when the plasma viscosity is high. We illu strate the problem with a comparison of hematocrit-adjusted blood from type 1 diabetes patients and controls in which it is found the apparent and rel ative viscosities at a true shear rate of 0.20 s(-1) are lower in the patie nt samples than age matched controls, in spite of reports that aggregation is increased in such populations. Because the plasma viscosities of the pat ients were higher on average than controls, we performed a series of experi ments to examine the effect of plasma protein concentration and viscosity o n normal blood viscosity. Dilution or concentration by ultrafiltration of a utologous plasma and viscosity measurements at low shear on constant hemato crit red cell suspensions showed (a) suspension viscosity at 0.25 and 3 s(- 1) increased monotonically with plasma protein concentration and viscosity but (b) the relative viscosity increased, in concert with the microscopic a ggregation grade, up to a viscosity of approximately 1.25 mPa-s but above t his the value the relative viscosity no longer increased as the degree of a ggregation increased in concentrated plasmas. It is suggested that in order to reduce cell deformation effects in hyperviscous pathological plasmas, p atient and control plasmas should be systematically diluted before hematocr it is adjusted and rheological measurements are made. True shear rates shou ld be calculated. Comparison of relative viscosities at low true shear rate s appears to allow the effects of red cell aggregation to be distinguished by variable shear rate viscometry in clinical blood samples.