do test the viscosity of whole blood but are only able to measure high shear viscosity (at a shear rate of 90 s-1). Systolic blood viscosity (high shear) and diastolic blood viscosity (low shear) are reported for easy clinical intepretation. Meridian Valley Laboratory uses an automated scanning capillary viscometer that acquires the complete viscosity profile for each blood sample tested. Because the blood cells are removed prior to testing, plasma and serum viscosity measurements cannot provide data that are physiologically meaningful and fail to capture the dynamic changes of blood flowing in the body. The viscosity of blood is changing dynamically during each cardiac cycle because the red blood cells aggregate during diastole. Inside the body, blood experiences high and low shear rates during each cardiac cycle-from peak-systole (high shear) to end-diastole (low shear). Serum and plasma are Newtonian fluids like water, which means that no matter how fast these fluids flow their viscosity does not changes (see the figure below). The phenomenon is known as the shear-thinning, non-Newtonian behavior of whole blood. When it moves slowly during end-diastole it is thicker and stickier. When blood moves quickly as in peak-systole, it is thinner. For convenience sake, we can think of shear rate simply as blood flow velocity. Shear rate is the velocity divided by the vessel diameter. Whole blood behaves as a non-Newtonian fluid, which means that its viscosity changes as a function of shear rate. In order to quantify the actual flow resistance of blood, it is necessary to measure whole blood viscosity. Whole blood viscosity is the ability of the blood to flow, which determines the burden of work to the heart in pumping blood, the physical injury that blood can cause against the inner lining of the arteries, and the delivery of oxygen to the organ systems. In particular, the red blood cells, their proportion to plasma volume, and the deformability (flexibility) of the red blood cells have a profound influence on actual blood viscosity and flow resistance. However, because the cellular content of blood has been removed prior to testing viscosity, plasma and serum cannot provide any insights on the actual flow resistance of a patient’s blood sample. Plasma and serum viscosity is also used in a number of clinical diagnoses such as Waldenström macroglobulinemia. Plasma and serum viscosity tests are useful to detect an acute phase reaction or severe inflammatory response such as occurs in sepsis. Serum viscosity is tested using a rotating or capillary viscometer and is marginally lower than plasma because the clotting factors have been removed. Then, after coagulation, the specimen is centrifuged to separate the serum from the clotted blood. Serum is similar to plasma, except that the blood is drawn into a collection tube without anti-coagulant and allowed to clot for about 20 minutes. Plasma viscosity is tested using a rotating or capillary viscometer using the separated plasma only. The plasma contains the dissolved proteins, clotting factors, and other suspended materials in water. Blood is drawn into a collection tube that contains an anti-coagulant (EDTA or citrate) and placed in a centrifuge to separate the cells from the plasma. Plasma is typically about 55% of the blood volume. Plasma differs from whole blood in that the cellular material has been removed by centrifugation. serum or plasma, and in addition, explains the critical importance of obtaining a complete viscosity profile and not just the systolic viscosity measurement. This white paper describes the difference between viscosity measurements for whole blood vs. Additionally, a very select few number of clinical reference labs claim to provide whole blood viscosity testing, however these tests are for systolic blood viscosity-high shear rate viscosity only. There are many reference laboratories and pathology departments of hospitals that can provide clinical viscosity testing, however these tests are for serum and plasma viscosity, not whole blood. Meridian Valley Laboratory is the only clinical reference lab that provides a comprehensive blood viscosity profile, which includes both systolic and diastolic blood viscosity measurements.
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