At a Glance
Why Get Tested?
To determine the proportion of your blood that is made up of red blood cells (RBCs), thus to screen for, help diagnose, or monitor conditions that affect RBCs; as part of a routine health examination or if your doctor suspects that you have anemia or polycythemia
When to Get Tested?
With a test for hemoglobin or as part of a complete blood count (CBC) during a routine health exam or when your doctor suspects you have a condition such as anemia (low hematocrit) or polycythemia (high hematocrit); at regular intervals to monitor a disorder that affects RBCs and to evaluate treatment effectiveness
A blood sample drawn from a vein in your arm or by a fingerstick (children and adults) or heelstick (newborns)
Test Preparation Needed?
The Test Sample
What is being tested?
Blood consists of a few types of cells, including red blood cells (RBCs), white blood cells (WBCs), and platelets, suspended in a fluid portion called plasma. A hematocrit is a test that measures the proportion of a person's blood that is made up of RBCs. It is a ratio of the volume of red blood cells to the volume of whole blood. The value is expressed as a percentage or fraction. For example, a hematocrit value of 40% means that there are 40 milliliters of red blood cells in 100 milliliters of blood.
The hematocrit is a fairly quick and simple way of evaluating a person's red blood cells. It is often performed in conjunction with a hemoglobin level and is also one component of the complete blood count (CBC), a test that is often used in the general evaluation of a person's health.
RBCs are produced in the bone marrow and are released into the bloodstream after they mature. They typically make up roughly 37% to 49% of the volume of blood. RBCs contain hemoglobin, a protein that binds to oxygen. The primary function of RBCs is to carry oxygen from the lungs to the tissues and organs of the body. They also transport a small portion of carbon dioxide, a byproduct of cell metabolism from tissues and organs back to the lungs, where it is expelled. The bone marrow must continually produce new RBCs to replace those that age and degrade or are lost through bleeding. The typical lifespan of an RBC is 120 days. There are a number of conditions that can affect either the production of new RBCs or the lifespan of those in circulation or that result in significant bleeding.
The hematocrit reflects both the number of red cells and their volume (mean corpuscular volume or MCV). If the size of the red cells decreases, so will the hematocrit and vice versa. In general, the hematocrit will rise when the number of red blood cells increases and the hematocrit will fall to less than normal when there is a drop in production of RBCs by the bone marrow, an increase in the destruction of RBCs, or if blood is lost due to bleeding. A drop in the RBC count, hemoglobin and hematocrit can result in anemia, a condition in which the body does not have the capacity to deliver enough oxygen to tissues and organs, causing fatigue and weakness. If too many RBCs are produced (resulting in increased hematocrit), then the blood can become thickened, causing sluggish blood flow and related problems.
How is the sample collected for testing?
A sample is obtained by drawing blood through a needle placed in a vein in the arm or by a fingerstick (for children and adults) or a heelstick (for newborns).
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
Sources Used in Current Review
Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. McPherson R, Pincus M, eds. Philadelphia, PA: Saunders Elsevier: 2007, Chap 31, Pp 459-460.
Kasper DL, Braunwald E, Fauci AS, Hauser SL, Longo DL, Jameson JL eds (2005). Harrison's Principles of Internal Medicine, 16th Edition, McGraw Hill, Pp 329-336.
Pagana K, Pagana T. Mosby's Manual of Diagnostic and Laboratory Tests. 3rd Edition, St. Louis: Mosby Elsevier; 2006, Pp 296-300.
Harmening D. Clinical Hematology and Fundamentals of Hemostasis. Fifth Edition, F.A. Davis Company, Piladelphia, 2009, Pp 82-85,771-773.
(Feb 9 2010) Dugdale D. Hematocrit. Medline Plus Medical Encyclopedia. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003646.htm. Accessed January 2012.
(December 2005) Mayo Reference Services. How to interpret and pursue an abnormal complete blood cell count in adults. Vol. 30 No. 12. PDF available for download at http://www.mayomedicallaboratories.com/media/articles/communique/mc2831-1205.pdf through http://www.mayomedicallaboratories.com. Accessed January 2012.
(March 1, 2011) National Heart, Lung and Blood Institute. What is Polycythemia vera? Available online at http://www.nhlbi.nih.gov/health/public/blood/index.htm through http://www.nhlbi.nih.gov. Accessed Jan 2012.
(Aug 1, 2010) National Heart, Lung and Blood Institute. Anemia. Available online at http://www.nhlbi.nih.gov/health/health-topics/topics/anemia/ through http://www.nhlbi.nih.gov. Accessed Jan 2012.
(November 4, 2011) Maarkaron J. Anemia. Medscape Reference article. Available online at http://emedicine.medscape.com/article/198475-overview through http://emedicine.medscape.com. Accessed Jan 2012.
(May 26, 2011) Kahsai D. Emergent Management of Acute Anemia. Medscape Reference article. Available online at http://emedicine.medscape.com/article/780334-overview#a1 through http://emedicine.medscape.com. Accessed Jan 2012.
(August 26, 2011) Harper J. Pediatric Megaloblastic Anemia. eMedicine article. Available online at http://emedicine.medscape.com/article/959918-overview through http://emedicine.medscape.com. Accessed Jan 2012.
(June 8, 2011) Artz A. Anemia in Elderly Persons. eMedicine article. Available online at http://emedicine.medscape.com/article/1339998-overview through http://emedicine.medscape.com. Accessed Jan 2012.
Riley R, et.al. Automated Hematologic Evaluation. Medical College of Virginia, Virginia Commonwealth University. Available online at http://www.pathology.vcu.edu/education/PathLab/pages/hematopath/pbs.html#Anchor-Automated-47857 through http://www.pathology.vcu.edu. Accessed Jan 2012.
Sources Used in Previous Reviews
Thomas, Clayton L., Editor (1997). Taber's Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].
Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosby's Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO.
Wu, A. (2006). Tietz Clinical Guide to Laboratory Tests, Fourth Edition. Saunders Elsevier, St. Louis, Missouri. Pp 514-517.