Hematocrit

DEFINITION ‎
The hematocrit is a test that measures the percentage of blood that is comprised of red blood cells.
It is the relative measure of red cell present in a whole blood expressed in percentage.
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 B12and 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, 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 hemo concentration. 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 guideto 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, known as venipuncture. When performing a finger puncture, the first drop of blood should be wiped away because it dilutes the sample with tissue fluid. A phlebotomist, clinician, lab technician/scientist 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.
MEASUREMENT AND ESTIMATION
The hematocrit may be measured manually by centrifugation.
Specimen
- 2.5 - 4.0ml of EDTA blood
- Heparinized finger prick blood
Procedure and Materials‎
To perform a hematocrit using the micro-hematocrit method, the following materials are required.
- Capillary tubes, plain or heparinized
- Modeling clay sealant
- Micro-hematocrit centrifuge
- Microhematocrit reader
Procedure
To perform the micro-hematocrit method, you should follow the steps listed below (we all used to this anyway)
- Fill the capillary tube two-thirds to three-quarters full with well-mixed, anricoagulated venous blood or fingertip blood. (For fingertip blood use heparinized tubes, and invert several times to mix.)
- Seal one end of the tube with clay/sealant/plasticine.
- Place the filled tube in the micro-hematocrit centrifuge, with the plugged end away from the center of the centrifuge.
- Centrifuge at a preset speed of 10,000 to 12,000 rpm for 5 minutes. If the hematocrit (PCV) exceeds 50 percent, centrifuge for an additional 3 minutes.
5. Place the tube in the microhematocrit reader.
Read the hematocrit by following the manufacturer's instructions on the micro-hematocrit reading device.
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 micro-hematocrit 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.
FACTORS AND ANALYTICAL ERRORS AFFECTING HEMATOCRIT
- Evaporation
- Volume of blood
- Time and speed of centrifugation
- Nature and type of sealant ‎
AFTER CARE‎
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 awhileis 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 Le Fever - Handbook of Laboratory and Diagnostic Tests 4th Ed. Upper Saddle River, NJ: Prentice Hall, 2001
- Kjeldsberg, Carl R - Practical Diagnosis of Hematologic Disorders 3rd Ed Chicago: ASCP Press, 2000‎