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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. 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. This white paper describes the difference between viscosity measurements for whole blood vs. serum or plasma, and in addition, explains the critical importance of obtaining a complete viscosity profile and not just the systolic viscosity measurement.
Whole Blood vs. Plasma and Serum
Plasma differs from whole blood in that the cellular material has been removed by centrifugation. Plasma is typically about 55% of the blood volume. 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. The plasma contains the dissolved proteins, clotting factors, and other suspended materials in water. Plasma viscosity is tested using a rotating or capillary viscometer using the separated plasma only.
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. Then, after coagulation, the specimen is centrifuged to separate the serum from the clotted blood. Serum viscosity is tested using a rotating or capillary viscometer and is marginally lower than plasma because the clotting factors have been removed. Plasma and serum viscosity tests are useful to detect an acute phase reaction or severe inflammatory response such as occurs in sepsis.
Plasma and serum viscosity is also used in a number of clinical diagnoses such as Waldenström macroglobulinemia. 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. 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. 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 order to quantify the actual flow resistance of blood, it is necessary to measure whole blood viscosity.
Newtonian vs. Non-Newtonian
Whole blood behaves as a non-Newtonian fluid, which means that its viscosity changes as a function of shear rate. Shear rate is the velocity divided by the vessel diameter. For convenience sake, we can think of shear rate simply as blood flow velocity. When blood moves quickly as in peak-systole, it is thinner. When it moves slowly during end-diastole it is thicker and stickier. This is because the red cells aggregate. The phenomenon is known as the shear-thinning, non-Newtonian behavior of whole blood.
Serum and plasma are Newtonian fluids like water, which means that no matter how fast these fluids flow their viscosity does not changes.
Inside the body, blood experiences high and low shear rates during each cardiac cycle—from peak-systole (high shear) to end-diastole (low shear). The viscosity of blood is changing dynamically during each cardiac cycle because the red blood cells aggregate during diastole. 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.