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Red blood cell count
Red Blood Cell Count (RBC)
The red blood cell (RBC) count, also coiled an erythrocyte count, is part of a complete blood count. It's used to detect the number of RBCs in a microliter, or cubic millimeter (mm3), of whole blood.
Purpose
- To provide data for calculating mean corpuscular volume and mean corpuscular hemoglobin, which reveal RBC size and hemoglobin content.
- To support other hematologic tests for diagnosing anemia or polycythemia.
Patient preparation
- Explain to the patient that this test is used to evaluate the number of RBCs and to detect possible blood disorders.
- Tell him that a blood sample will be taken. Explain who will perform the venipuncture and when.
- Reassure him that drawing a blood sample will take less than 3 minutes.
- Explain that he may feel slight discomfort from the tourniquet pressure and the needle puncture.
- If the patient is a child, explain to him (if he's old enough) and his parents that a small amount of blood will be taken from the finger or earlobe.
- Inform the patient that food or fluids need not be restricted before the test.
Procedure and posttest care
- For adults and older children, draw venous blood into a 7-ml lavender-top tube.
- For younger children, collect capillary blood in a micro collection device.
- If a hematoma develops at the venipuncture site, apply warm soaks.
Precautions
- Completely fill the collection tube.
- Invert the tube gently several times to mix the sample and the anticoagulant.
- Handle the sample gently to prevent hemolysis.
Reference values
Normal RBC values vary, depending on the type of sample and on the patient's age and sex, as follows:
- adult males: 4.5 to 5.5 million RBCs/u1 of venous blood.
- adult females: 4 to 5 million RBCs/u1 of venous blood.
- children ages 1 to 6: 3.9 to 5.3 million RBCs/u1 of venous blood.
- full-term infants: 4.1 to 6.1 million RBCs/u1 of capillary blood at birth, decreasing to 3.8 to 5.6 million RBCs/u1 at age 2 months, and increasing slowly thereafter.
Normal values may exceed these levels in patients living at high altitudes.
Abnormal findings
An elevated RBC count may indicate absolute or relative polycythemia. A depressed count may indicate anemia, fluid overload, or hemorrhage beyond 24 hours. Further tests, such as stained cell examination, hematocrit, hemoglobin, red cell indices, and white cell studies, are needed to confirm the diagnosis.
Interfering factors
- Failure to use the proper anticoagulant or to adequately mix the sample and the anticoagulant.
- Hemoconcentration due to prolonged tourniquet constriction.
- Hemodilution due to drawing the sample from the same ann used for I. V. infusion of fluids.
- High white blood cell count (falsehigh test results in semi automated and automated counters).
- Diseases that cause RBCs to agglutinate or form rouleaux (false decrease).
- Hemolysis due to rough handling of
the sample or drawing the blood through a small-gauge needle for venipuncture.
- Pregnancy.
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