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Definition

The hematocrit is a test that measures the volume of blood by the percentage that is comprised of red blood cells. The hematocrit is also called the packed red cell volume because classically it is measured by centrifuging the blood in a capillary tube.

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, Addison's disease, and dehydration. These conditions reduce the volume of plasma water causing a relative increase in RBCs or hemoconcentration. An elevated hematocrit may also be caused by an absolute increase in blood cells called polycythemia. This may be secondary to hypoxia or the result of a proliferation of blood forming cells in the bone marrow (polycythemia vera).

Transfusion decisions are based upon the results of laboratory tests, including the hematocrit. Generally, transfusion is not considered necessary if the hematocrit is below 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 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, care should be taken to avoid hemolysis, as this will invalidate test results. Certain drugs such as penicillin and chloramphenicol may decrease the hematocrit, while glucose levels above 400 mg/dL are known to elevate results. Blood should be collected in heparin or

EDTA (edetic acid) and measured within six hours to avoid RBC (red blood cell) swelling. Prolonged use of the tourniquet during collection will increase the hematocrit. Excess EDTA caused by an incomplete filling of the tube will falsely lower the hematocrit. Blood for hematocrit may be collected either by finger puncture or 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 standard precautions for the prevention of transmission of blood-borne pathogens.

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

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KEY TERMS

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Anemia—A lack of oxygen carrying capacity commonly caused by a decrease in red blood cell number, size, or function.

Hematocrit—The volume of blood occupied by the red blood cells expressed in percent.

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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 mass and are forced to the bottom of the tube. The WBCs and platelets form a thin layer between the RBCs and the plasma called the buffy coat, 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. The counter measures the RBCs by impedence. A RBC displaces electrolyte in the counting aperture causing a voltage pulse proportional to the cell size. The instrument calculates the mean cell volume (MCV) from the voltage pulses. The MCV is multiplied by the RBC count to give the hematocrit. Whole blood electrolyte analyzers measure the hematocrit using a conductivity electrode. The electrical conductivity of the sample is inversely related to the hematocrit after correcting for the sodium concentration. 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 should be treated accordingly.

Complications

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

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%

Health care team roles

Laboratory scientists perform hematocrit tests using manual or automated procedures. Critically high or low levels should be immediately called to the attention of the patient's nurse or doctor. Nurses should bring high or low hematocrit levels to the attention of the patient's physician, and should also report any signs and symptoms that could be associated with the increase or decrease such as medications, excessive thirst, tachycardia, low blood pressure, weakness, etc.

Victoria E. DeMoranville