The main function of the RBC (erythrocyte) is to carry oxygen from the lungs to the body tissues and to transfer carbon dioxide from the tissues to the lungs. This process is achieved by means of the Hb in the RBCs, which combines easily with oxygen and carbon dioxide and gives arterial blood a bright red appearance. To enable use of the maximal amount of Hb, the RBC is shaped like a biconcave disk—this affords more surface area for the Hb to combine with oxygen. The cell also is able to change its shape when necessary to allow for passage through the smaller capillaries.

The RBC count, an important measurement in the evaluation of anemia or polycythemia, determines the total number of erythrocytes in a microliter (cubic millimeter) of blood.

See Table 2.4.

1. Obtain 5 mL of whole blood in a lavender-topped tube (with EDTA). Label the specimen with the patient’s name, date and time of collection, and test(s) ordered. Place the specimen in a biohazard bag.

2. Automated electronic devices are generally used to determine the number of RBCs.

3. Note patient age and time of day on the laboratory slip.

1. Decreased RBC values occur in: (This list is not meant to be all inclusive.)

   1. Anemia, a condition in which there is a reduction in the number of circulating erythrocytes, the amount of Hb, or the volume of packed cells (Hct). Anemia is associated with cell destruction, blood loss, or dietary insufficiency of iron or of certain vitamins that are essential in the production of RBCs. See Chart 2.1 (later in this chapter) for a classification of anemias based on their underlying mechanisms and the test for reticulocyte count for a discussion of the purpose and clinical implications of the reticulocyte count.

   2. Disorders such as:

      1. Hodgkin disease and other lymphomas

      2. Multiple myeloma, myeloproliferative disorders, leukemia

      3. Acute and chronic hemorrhage

      4. Lupus erythematosus

      5. Addison disease

      6. Rheumatic fever

      7. Subacute endocarditis, chronic infection

2. Erythrocytosis (increased RBC count) occurs in:

   1. Primary erythrocytosis

      1. Polycythemia vera (myeloproliferative disorder)

      2. Erythremic erythrocytosis (increased RBC production in bone marrow)

   2. Secondary erythrocytosis

      1. Kidney disease

      2. Extrarenal tumors

      3. High altitude

      4. Pulmonary disease

      5. Cardiovascular disease

      6. Alveolar hypoventilation

      7. Hemoglobinopathy

      8. Tobacco, carboxyhemoglobin

   3. Relative erythrocytosis (decrease in plasma volume)

      1. Dehydration (vomiting, diarrhea)

      2. Gaisböck syndrome

Pretest Patient Care

1. Explain test purpose and procedure. Assess for signs/symptoms of fatigue, shortness of breath, weakness, tachycardia, and pallor of skin and mucous membranes.

2. Refer to standard pretest care for CBC and differential count.

3. Have the patient avoid extensive exercise, stress, and excitement before the test. These cause elevated counts of doubtful clinical value.

4. Avoid overhydration or dehydration, if possible—either causes invalid results. If patient is receiving IV fluids or therapy, note on requisition.

5. Note any medications the patient is taking.

6. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

1. Posture: When a blood sample is obtained from a healthy person in a recumbent position, the RBC count is 5% lower. (If the patient is anemic, the count will be lower still.)

2. Dehydration: Hemoconcentration in dehydrated adults (caused by severe burns, untreated intestinal obstruction, severe persistent vomiting, or diuretic abuse) may obscure significant anemia.

3. Age: The normal RBC count of a newborn is higher than that of an adult, with a rapid drop to the lowest point in life at 2–4 months. The normal adult level is reached at age 14 years and is maintained until old age, when there is a gradual drop (see Table 2.4).

4. Falsely high counts may occur because of prolonged venous stasis during venipuncture.

5. Stress can cause a higher RBC count.

6. Altitude: The higher the altitude, the greater the increase in RBC count. Decreased oxygen content of the air stimulates the RBC count to rise (erythrocytosis).

7. Pregnancy: There is a relative decrease in RBC count when the body fluid increases in pregnancy, with the normal number of erythrocytes becoming more diluted.

8. There are many drugs that may cause decreased or increased RBC count. see Appendix E for drugs that affect test outcomes.

9. The EDTA blood sample tube must be at least three fourths filled or values will be invalid because of cell shrinkage caused by the anticoagulant.

10. The blood sample must not be clotted (even slightly) or the values will be invalid.