hematocrit

Definition

The hematocrit measures how much space in the blood is occupied by red blood cells. It is useful when evaluating a person for anemia.

Description

Blood drawn from a fingerstick is often used for hematocrit testing. The blood fills a small tube, which is then spun in a small centrifuge. As the tube spins, the red blood cells go to the bottom of the tube, the white blood cells cover the red in a thin layer called the buffy coat, and the liquid plasma rises to the top. The spun tube is examined for the line that divides the red cells from the buffy coat and plasma. The height of the red cell column is measured as a percent of the total blood column. The higher the column of red cells, the higher the hematocrit.

The hematocrit test can also be done on an automated instrument as part of a complete blood count. It is also called Packed Red Cell Volume or Packed Cell Volume, or abbreviated as Hct or Crit. The test is covered by insurance when medically necessary. Results are usually available the same or following day.

— Nancy J. Nordenson

1. The percentage by volume of packed red blood cells in a given sample of blood after centrifugation.
2. A centrifuge used to determine the volume of blood cells and plasma in a given sample of blood.

[HEMATO- + Greek kritēs, judge (from krīnein, to judge).]

Definition

The hematocrit is a test that measures the percentage of blood that is comprised of red blood cells.

Purpose

The hematocrit is used to screen for anemia, or is measured on a person to determine the extent of anemia. An anemic person has fewer or smaller than normal red blood cells. A low hematocrit, combined with other abnormal blood tests, confirms the diagnosis. The hematocrit is decreased in a variety of common conditions including chronic and recent acute blood loss, some cancers, kidney and liver diseases, malnutrition, vitamin B₁₂ and folic acid deficiencies, iron deficiency, pregnancy, systemic lupus erythematosus, rheumatoid arthritis and peptic ulcer disease. An elevated hematocrit is most often associated with severe burns, diarrhea, shock, Ad dison's disease, and dehydration, which is a decreased amount of water in the tissues. These conditions reduce the volume of plasma water causing a relative increase in RBCs, which concentrates the RBCs, called hemoconcentration. An elevated hematocrit may also be caused by an absolute increase in blood cells, called polycythemia. This may be secondary to a decreased amount of oxygen, called hypoxia, or the result of a proliferation of blood forming cells in the bone marrow (polycythemia vera).

Critically high or low levels should be immediately called to the attention of the patient's nurse or doctor. Transfusion decisions are based on the results of laboratory tests, including the hematocrit. Generally, transfusion is not considered necessary if the hematocrit is above 21%. The hematocrit is also used as a guide to how many transfusions are needed. Each unit of packed red blood cells administered to an adult is expected to increase the hematocrit by approximately 3% to 4%.

Precautions

Fluid volume in the blood affects hematocrit values. Accordingly, the blood sample should not be taken from an arm receiving IV fluid or during hemodialysis. It should be noted that pregnant women have extra fluid, which dilutes the blood, decreasing the hematocrit. Dehydration concentrates the blood, which increases the hematocrit.

In addition, certain drugs such as penicillin and chloramphenicol may decrease the hematocrit, while glucose levels above 400 mg/dL are known to elevate results. Blood for hematocrit may be collected either by finger puncture, or sticking a needle into a vein, called venipuncture. When performing a finger puncture, the first drop of blood should be wiped away because it dilutes the sample with tissue fluid. A nurse or phlebotomist usually collects the sample following cleaning and disinfecting the skin at the site of the needle stick.

Description

Blood is made up of red blood cells, white blood cells (WBCs), platelets, and plasma. A decrease in the number or size of red cells also decreases the amount of space they occupy, resulting in a lower hematocrit. Conversely, an increase in the number or size of red cells increases the amount of space they occupy, resulting in a higher hematocrit. Thalassemia minor is an exception in that it usually causes an increase in the number of red blood cells, but because they are small, it results in a decreased hematocrit.

The hematocrit may be measured manually by centrifugation. A thin capillary tube called a microhematocrit tube is filled with blood and sealed at the bottom. The tube is centrifuged at 10,000 RPM (revolutions per minute) for five minutes. The RBCs have the greatest weight and are forced to the bottom of the tube. The WBCs and platelets form a thin layer, called the buffy coat, between the RBCs and the plasma, and the liquid plasma rises to the top. The height of the red cell column is measured as a percent of the total blood column. The higher the column of red cells, the higher the hematocrit. Most commonly, the hematocrit is measured indirectly by an automated blood cell counter. It is important to recognize that different results may be obtained when different measurement principles are used. For example, the microhematocrit tube method will give slightly higher results than the electronic methods when RBCs of abnormal shape are present because more plasma is trapped between the cells.

Aftercare

Discomfort or bruising may occur at the puncture site. Pressure to the puncture site until the bleeding stops reduces bruising; warm packs relieve discomfort. Some people feel dizzy or faint after blood has been drawn, and lying down and relaxing for awhile is helpful for these people.

Risks

Other than potential bruising at the puncture site, and/or dizziness, there are no complications associated with this test.

Normal Results

Normal values vary with age and sex. Some representative ranges are:

• at birth: 42-60%
• six to 12 months: 33-40%
• adult males: 42-52%
• adult females: 35-47%

Resources

Books

Chernecky, Cynthia C. and Barbara J. Berger. Laboratory Tests and Diagnostic Procedures. 3rd ed. Philadelphia: W. B. Saunders Company, 2001.

Kee, Joyce LeFever. Handbook of Laboratory and DiagnosticTests. 4th ed. Upper Saddle River, NJ: Prentice Hall, 2001.

Kjeldsberg, Carl R. Practical Diagnosis of Hematologic Disorders. 3rd ed. Chicago: ASCP Press, 2000.

Organizations

American Association of Blood Banks. 8101 Glenbrook Road, Bethesda, Maryland 20814. (301) 907-6977. Fax: (301) 907-6895. http://www.aabb.org.

Other

Uthman, Ed. Blood Cells and the CBC. 2000 [cited February 17, 2003]. http://web2.iadfw.net/uthman/blood_cells.html.

— Victoria E. DeMoranville
Mark A. Best

(packed-cell volume), 1. the percentage of the total blood volume composed of red blood cells (erythrocytes). Normal values are 42% to 45%. n 2. the percentage of the total volume of a blood sample that is taken up by the red blood cells. Normal values: children, 32% to 65%; adult men, 42% to 53%; adult women, 38% to 46%.

Blood has a liquid component (plasma) and a particulate component (blood cells). The denser blood cells (most of which are red blood cells) will settle in a tube, particularly if the blood is spun in a device called a centrifuge. The fraction of the resultant column composed of red blood cells relative to the entire column is the hematocrit, which normally is in the range of 40 percent. The upper portion of the separated blood is yellowish plasma. Red blood cells carry oxygen, and blood with a low hematocrit (e.g., 20%) leaves the tissues relatively oxygen-starved and weak. A high hematocrit (e.g., 70%) produces problems as well, but is uncommon.

(SEE ALSO: Hemoglobin)

Bibliography

Corash, L. (1995). "Laboratory Hematology: Methods for the Analysis of Blood." In Blood: Principles and Practice of Hematology, eds. R. I. Handin, S. E. Lux, and T. P. Stossel. Philadelphia, PA: J. B. Lippincott Company.

— KENNETH R. BRIDGES

The volume percentage of erythrocytes in whole blood; also, the apparatus or procedures used in its determination. The hematocrit (which means, literally, ‘to separate blood’) is determined by centrifuging a blood sample to separate the cellular elements from the plasma; the results of the test indicate the ratio of cell volume to plasma volume (packed cell volume, PCV) and are expressed as milliliters of packed cells per 100 ml of blood, or in volumes per 100 ml. The hematocrit, in conjunction with other hematological tests, provides information about the size, functioning capacity and number of erythrocytes. See also wintrobe method.

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The hematocrit (Ht or HCT) or packed cell volume (PCV) or erythrocyte volume fraction (EVF) is the proportion of blood volume that is occupied by red blood cells. It is normally about 48% for men and 38% for women.^[1] It is considered an integral part of a person's complete blood count results, along with hemoglobin concentration, white blood cell count, and platelet count.

In mammals, hematocrit is independent of body size.

The term "hematocrit" (British English: haematocrit) was coined in 1903. Its roots stem from the Greek words hema (Gr αίμα) - blood, and krites (Gr κριτής), judge - meaning to gauge or judge the blood.

Contents 1 Measurement methods 2 Elevated hematocrit 3 Lowered hematocrit 4 See also 5 References

Measurement methods

Packed cell volume diagram

The packed cell volume (PCV) can be determined by centrifuging heparinized blood in a capillary tube (also known as a microhematocrit tube) at 10,000 RPM for five minutes.^[2] This separates the blood into layers. The volume of packed red blood cells, divided by the total volume of the blood sample gives the PCV. Because a tube is used this can be calculated by measuring the lengths of the layers.

With modern lab equipment, the hematocrit is calculated by an automated analyzer and not directly measured. It is determined by multiplying the red cell count by the mean cell volume. The hematocrit is slightly more accurate as the PCV includes small amounts of blood plasma trapped between the red cells. An estimated hematocrit as a percentage may be derived by tripling the hemoglobin concentration in g/dL and dropping the units.^[3] The hemoglobin level is the measure used by blood banks.^{[clarification needed][citation needed]}

There have been cases where the blood for testing was inadvertently drawn proximal to an intravenous line that was infusing packed red cells or fluids. In these situations, the hemoglobin level in the blood sample will not be the true level for the patient because the sample would contain a large amount of the infused material rather than what is diluted into the circulating whole blood. That is, if packed red cells are being supplied, the sample will contain a large amount of those cells and the hematocrit will be artificially very high. Conversely, if saline or other fluids are being supplied, the blood sample would be diluted and the hematocrit will be artificially low.

Elevated hematocrit

In cases of dengue fever a high hematocrit is a danger sign of an increased risk of dengue shock syndrome.

Polycythemia vera (PV), a myeloproliferative disorder in which the bone marrow produces excessive numbers of red cells, is associated with elevated hematocrit.

Chronic obstructive pulmonary disease (COPD) and other pulmonary conditions associated with hypoxia may elicit an increased production of red blood cells. This increase is mediated by the increased levels of erythropoietin by the kidneys in response to hypoxia.

Professional athletes' hematocrit levels are measured as part of tests for blood doping or Erythropoietin (EPO) use; the level of hematocrit in a blood sample is compared with the long-term level for that athlete (to allow for individual variations in hematocrit level), and against an absolute permitted maximum (which is based on maximum expected levels within the population, and the hematocrit level which causes increased risk of blood clots resulting in strokes or heart attacks).

Steroid use can also increase the amount of RBC's and therefore impact the hematocrit.

If a patient is dehydrated, the hematocrit may be elevated. Repeat testing after adequate hydration therapy will usually result in a more reliable result.

Lowered hematocrit

Lowered hematocrit can imply significant hemorrhage (for example, in an ectopic pregnancy.)

The mean corpuscular volume (MCV) and the red cell distribution width (RDW) can be quite helpful in evaluating a lower-than-normal hematocrit, because it can help the clinician determine whether blood loss is chronic or acute. The MCV is the size of the red cells and the RDW is a relative measure of the variation in size of the red cell population. A low hematocrit with a low MCV with a high RDW suggests a chronic iron-deficient erythropoiesis, but a normal RDW suggests a blood loss that is more acute, such as a hemorrhage.

Groups of individuals who are at risk for developing anemia include:

• infants who may not have adequate iron intake
• children going through a rapid growth spurt, during which the iron available cannot keep up with the demands for a growing red cell mass
• women in childbearing years who have an excessive need for iron because of blood loss during menstruation
• pregnant women, in whom the growing fetus creates a high demand for iron.
• patients with chronic kidney disease, as their kidneys no longer secrete sufficient levels of the hormone erythropoietin, which stimulates red blood cell production by the bone marrow.

See also

• Anemia
• Blood tests
• Hematology
• Blood doping

References

1. ^ Purves, William K.; David Sadava, Gordon H. Orians, H. Craig Heller (2004). Life: The Science of Biology (7th ed.). Sunderland, Mass: Sinauer Associates. pp. 954. ISBN 0-7167-9856-5. 
2. ^ "Hematocrit". Encyclopedia of Surgery: A Guide for Patients and Caregivers. http://www.surgeryencyclopedia.com/Fi-La/Hematocrit.html. 
3. ^ "Hematocrit (HCT) or Packed Cell Volume (PCV)". http://www.thedoctorslounge.net/hematology/labs/hematocrit.htm. 

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