Cellular content is removed from plasma prior to analysis, and in the case of serum, clotting factors have also been removed. Using an Ostwald tube or a rotational viscometer, the viscosity of plasma and serum can be easily quantified, and these methods are widely used in hospitals and clinical reference laboratories. Despite its U-shaped capillary design, the Ostwald tube is more primitive than rotational viscometers.
Plasma and serum viscosity tests are useful to detect an acute phase reaction or severe inflammatory response such as in sepsis. Plasma and serum viscosity are also used in a number of clinical diagnoses such as Waldenström macroglobulinemia. Because cellular content of blood is removed prior to testing viscosity (and, in the case of serum, clotting factors as well), plasma and serum viscosity tests do not provide a direct representation of a subject’s hemodynamic or hemorheologic status. Whole blood viscosity directly modulates the work of the heart, peripheral vascular resistance, endothelial shear stress, and microcirculatory perfusion. However such physiologic insights are not readily available from testing plasma or serum viscosity.
The red blood cells, their proportion to plasma volume, and the deformability (flexibility) of the red cells have a profound influence on actual blood viscosity and flow resistance. Whole blood viscosity is the inherent resistance of the blood to flow, which directly affects the work of the heart, endothelial shear stress (the frictional injury that blood causes against the inner lining of the arteries), and the delivery of oxygen to all tissues and organ systems. In order to quantify the actual flow resistance of blood, it is necessary to measure the viscosity of whole blood not plasma or serum alone.
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