Improving accuracy of co-axial viscometry.

Aggregation and sedimentation of the red cells in the viscometer makes it difficult to obtain good reproducibility at low shear rates. A simple method is described to obtain repeatable torque-time curves with the Contraves LS30. Before starting rotation, the blood sample is agitated by an up and down movement of the bob. This results in identical (within 1-2% of the actual amplitude) repeated tracings even at the lowest shear rates, even with extremely time-dependent samples. The subsequent torque-time curve of the disaggregated and unsettled samples provides an experimental basis for the quantitative description of blood viscoelasticity and for dynamic analysis of red cell aggregation: furthermore it results in more accurate measurements when comparing different samples and testing therapeutic effects. The method is applicable to any coaxial cylinder type viscometer with manual lowering of the bob.