Frequently Asked Questions

Have questions? Blood Flow Online offers expert answers to your inquiries about blood viscosity and related health topics. Bookmark this page for future reference and watch for updates. 

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Why is it important to know my whole blood viscosity?

A complete blood viscosity profile that includes high and low shear viscosity measurements is the only way to measure how thick and sticky your blood is. The health of every cell, tissue and organ in your body depends on life-sustaining blood flow to deliver oxygen and nutrients as well as remove carbon dioxide and wastes. Blood viscosity is a direct measure of the inherent resistance of blood to flow.

Whole blood viscosity also determines how hard your heart has to work to pump blood and how much friction (or shear stress) the blood causes against the inner lining of arteries. Endothelial shear stress, which is the tangential frictional force applied by the blood against the large arteries, is a major cause of atherosclerosis, the leading cause of death in the world.

Isn't it enough to know hematocrit, why do I need to know WBV?

Hematocrit, which is the percentage of blood volume made up of red blood cells, is an important factor influencing blood viscosity and plays a major role in determining how thick or thin your blood is. However hematocrit alone does not provide a complete picture of your blood’s ability to flow. Other factors, such as platelet count and activity, the concentration of proteins like fibrinogen, as well as lipid levels including triglyceride and LDL-cholesterol concentrations, can make the blood stickier and reduce its ability to flow. These factors increase red blood cell aggregation which is detected by a complete blood viscosity profile but not by hematocrit. 

Why aren't blood viscosity tests readily available?

A variety of blood viscosity tests are in fact available.

The viscosity of serum and plasma is commonly tested in hospitals and clinical laboratories throughout the U.S [LINK TO OUR PERSPECTIVE ON SERUM AND PLASMA TESTING].

Whole blood viscosity testing by a single-point rotating viscometer is also available at a number of laboratories, including ARUP (link to This test is performed at a mid-range shear rate of 90 s-1 using a Brookfield cone-and-plate viscometer.

Whole blood viscosity testing by a more advanced scanning capillary viscometer is also available through Meridian Valley Lab (link to, a specialty laboratory based in Seattle serving most U.S. states and Canada.

Why haven't I heard about whole blood viscosity before?

Most people haven’t heard of whole blood viscosity yet because measurement and reporting methods for whole blood viscosity were not standardized for many decades.

There are over 10,000 scientific papers published on blood viscosity, and blood viscosity has been shown to predict cardiovascular events like heart attacks and strokes. However most of this prior research was performed using single-point rotating viscometers which meant that researchers picked their own shear rate for testing and reporting blood viscosity. Often viscosity was tested and reported at different shear rates from one study to the next.

Even though clinical research demonstrated the importance of whole blood viscosity, comparison of data between studies was difficult because of the lack of standardization. Without uniformly established practices on selection of shear rates for blood viscosity tests, this diagnostic test is still not widely used.

Today, the newest capillary-type viscometers are able to scan the entire viscosity curve, making standardization of whole blood viscosity measurement more feasible. 

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