Hematocrit

Hematocrit is a measure which determines the volume of red cells in whole blood. It may be indicated in several ways on lab analysis: PCV (packed cell volume) or EVF (erythrocyte volume fraction). The measurement is significant in that it can indicate several factors, including blood loss, anemia, infection, or the presence of disease. Men and women have different normal values, with women generally around 40 percent and men at 45 percent.

The major constituent of red cells, known as erythrocytes, is a protein hemoglobin crucial to the survival of humans and all vertebrates because it delivers oxygen to all cells. A reduced volume of red cells, and consequentially oxygen, can cause not only fatigue but also vital organ damage and death if sustained (chronic) or sudden (as in trauma). Erythrocytes have biconcave surfaces which increases carbon dioxide and oxygen diffusion. Anomalies can reduce their efficiency.

Brief History

Marcello Malpighi, an anatomist in the seventeenth century, made preliminary findings of what we now know as erythrocytes. He called them red particles and described their flexibility some thirteen years prior to Antoni Van Leeuwenhoek’s 1674 publication noting "globules in blood." His observations were published in April of that year in the Philosophical Transactions of the Royal Society of London. Van Leeuwenhoek subsequently revealed another discovery about blood as he wrote that "those sanguineous globules in a healthy body must be very flexible and pliant if they are to pass through the small capillary veins and arteries, and that in their passage they change into an oval figure, reassuming their roundness when they come into a larger room."

Jan Swammerdam, a Dutch physician who frequently visited Leeuwenhoek was also involved in blood study. Though he learned much from the inventor, including how to use the glass capillary tube Leuwenhoek invented to view blood components, he had great disdain for Leuwenhoek because he lacked a formal education. Yet, it was Leeuwenhoek whose published work is credited with the discovery

In 1901, Austrian biologist and physician Karl Landsteiner introduced his discovery of the three major blood groups. Over subsequent years he and associates identified the Rhesus factor (in 1937) and identified the polio virus.

After the discovery of the hematocrit it was not until 1932 that a standard was pioneered by Max Mayer Wintrobe in New Orleans. This was the advent of the Wintrobe indices: a measurement of mean cell volume, mean cell hemoglobin, and mean cell hemoglobin concentration. The study of the hematocrit and its components has grown exponentially even from the days of Wintrobe. Technology in optics, analysis, and improved chemistry knowledge are one of the reasons for the leaps in detailed discoveries.

It was in 1959 that Max Perutz, a British molecular biologist, identified the structure of hemoglobin; the major hematocrit component. This discovery was made with the aid of X-ray crystallography. As technology advances, so does hematocrit study. Computer models now predict red blood cell flow, and that is just one small technological contribution.

Overview

Hematocrit and hemoglobin are almost always evaluated together. While total volume is significant, the hemoglobin needs to be in physiologic proportion. The percentage of RBC to plasma specifies whether the body is producing enough, too much, or too few of the cells. There are generalities for the amount of blood in the body, but actual volume is dependent on body mass. However, volume percentages are fairly consistent independent of size. The hematocrit serves as an indication of how many units of packed cells are required for transfusions. A rule of thumb is generally an increase in volume of 3 percent to 4 percent for each unit.

There is a distinction between the two terms used to describe an abnormally high hemoglobin count: Erythrocytosis refers simply to a higher proportion of erythrocytes to plasma, Polycytemia is the over production of the red blood cells.

Too many erythrocytes can indicate compensation for low oxygen levels as a result of heart or lung disease, reduced kidney function, reduced oxygen capacity of the RBCs, loss of blood plasma (commonly due to dehydration), or over production in the bone marrow. Some instances of a high red blood cell count are normal, as in the case of higher elevation where the available oxygen is lower than at sea level.

Low red blood cell counts are not always a sign of problems. Pregnancy often is accompanied by low erythrocytes (another reason for taking prenatal supplements as prescribed). Heavy menstrual bleeding is a common, and usually normal, etiology of blood loss. However, in most cases a lower than normal count requires investigation. The body may be destroying the cells faster than they can be produced, major blood loss is occurring, or RBC are not being produced due to illness or injury

Several common situations influence the hematocrit independent of disease: Blood glucose levels over 400 mg/dl elevate values, and certain medications, such as chloramphenicol and penicillin, can decrease values. Drawing blood from a limb in which an intravenous infusion is running will likely alter hematocrit values as well. Ignoring simple standard procedures, such as failing to wipe away the first drop of blood in a finger stick, will give an inaccurate reading, as the first drop is diluted from the fluid in the tissue.

Bibliography

Bessis, M., and D. Delpech. "Discovery of Red Blood Cells." Medical School of Virginia, n.d. Web. 30 Dec. 2015. http://www.med-ed.virginia.edu/courses/cell/resources/blooddisc.htm.

Bush, J. A., et al. "The Anemia of Infection: The Kinetics of Iron Metabolism in the Anemia Associated with Chronic Infection." Journal of Clinical Investigation 35.1 (1956): 89–97. Print.

Hoffman, Ronald, et al. Hematology: Basic Principles and Practice. Philadelphia: Elsevier Health Sciences, 2013. Print.

Lee, G. Richard., and Maxwell M. Wintrobe. Wintrobe's Clinical Hematology. Philadelphia: Lea and Febiger, 1993. Print.

"Malpighi, Swammerdam and Leeuwenhoek." JAMA: The Journal of the American Medical Association 285.15 (2001). Web. 30 Dec. 2015.

Valintine, William N. "Maxwell Mayer Wintrobe." National Academy of Science (1990): n. pag. Web. 30 Dec. 2015. http://www.nasonline.org/publications/biographical-memoirs/memoir-pdfs/wintrobe-m-m.pdf .

Walker, H. Kenneth, W. Dallas Hall, and J. Willis Hurst. Clinical Methods: The History, Physical, and Laboratory Examinations. Boston: Butterworths, 1990. Print.

Wintrobe, Maxwell M. Blood, Pure and Eloquent: A Story of Discovery, of People, and of Ideas. New York: McGraw-Hill, 1980. Print.