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What Is Hematocrit? - Definition, Normal Levels & Range

Instructor: Sarah Phenix
In this lesson, we will explore what a hematocrit test is, how it is run, and what it actually informs us about the patient. Furthermore, we will explore what the hematocrit does not indicate so that we can get a better understanding of its role as one of a collection of tests.

What is a Hematocrit?

Have you ever wondered what happens with that little vial of blood that is sometimes taken during a routine physical? Well, most of the time it is taken for something called a complete blood count (CBC), which includes a whole battery of tests, such as a count of your red blood cells, white blood cells and platelets, as well as something called a hematocrit.

A hematocrit is a blood test, which indicates what percentage of your whole blood volume is specifically made up of erythrocytes, or red blood cells (the oxygen carrying component of blood). What do I mean by whole blood volume, you ask? Well, you might already know that blood is made up of different living cells, collectively known as formed elements, which includes your erythrocytes, leukocytes, or white blood cells (the immune component of your blood), and platelets, or thrombocytes (the clotting component of your blood). Now, while about 45% of your whole blood content is composed of these formed elements, the other 55% is made up of a non-living component called plasma.

Cells found in Blood
Cells of Blood

Therefore, because blood has all of these different components, it's important that we be able to ascertain what portion of the whole blood volume is specifically made up by these vital oxygen-carrying erythrocytes. Too many or too few could be an indication that there are other bigger problems going on within that patient.

What Does a Hematocrit Show?

A hematocrit only indicates if a patient's volume of red blood cells falls within an established normal range. Now, this normal range varies by a few factors such as sex (the normal erythrocyte volume for men ranges from 40-54% erythrocytes while in women are 37-48% erythrocytes) and by species. Wait, what? Species too? Now, why would I even bring different species into this topic? Well, hematocrits aren't just run on humans, veterinarians run them as well. If you were to run a hematocrit on a sheep you would look for a normal range of around 25-45% packed erythrocytes.

Now, another thing that you should note is that these are ranges and that these ranges can be fairly vast (remember sheep can be anywhere from 25-45%, that's a huge 20% different). Why is this? Well, body size plays a large part in that and since there is such a large range in the weight of men as well as the weight of women (not to mention the weight range of sheep), the normal range was established to consider those factors. So, if you or I went to have a hematocrit run they would take into account your sex, weight, and even your age.

How is a Hematocrit Run?

Distribution of Cells After Being Spun in a Centrifuge
Blood Cell Distribution Post Centrifuge

Well, a sample of blood is collected and, from that, a thin glass tube, called a capillary tube, is filled and then placed into a centrifuge to be spun down. Now, because all of the cells of your blood have different weights, spinning them separates them into nice, clear columns of cells and fluid. Erythrocytes, weighing the most, collect in the bottom portion of the tube, while leukocytes and thrombocytes collect in the middle layer and plasma, being made up of molecules of electrolytes, water, and proteins, layers at the top of the tube. Once the blood is spun down, all that's left is a simple calculation where the length of the column of erythrocytes is measured. It is then divided by the length of the total column, and voilà, you have the percentage of your whole blood volume formed by your packed red blood cells.

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