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Introduction ⬇

A complete blood count (CBC), also referred to as a hemogram, consists of a WBC count, RBC count, Hb, Hct, RBC indices, and a platelet count. Table 2.2 contains the normal values for a CBC.

The CBC is a basic screening test and is one of the most frequently ordered laboratory procedures. The findings in the CBC give valuable diagnostic information about the hematologic and other body systems, prognosis, response to treatment, and recovery. The CBC consists of a series of tests that determine number, variety, percentage, concentrations, and quality of blood cells:

  1. WBC count: reports the total number of WBCs (leukocytes), which are part of the body’s immune response

  2. Differential WBC count (Diff): identifies specific patterns of WBCs by percentage of each cell type (see Differential White Blood Cell Count [Diff; Differential Leukocyte Count])

  3. RBC count: reports the total number of RBCs, which carry O2 from lungs to blood tissues and CO2 from tissue to lungs

  4. Hct: percentage of RBCs’ mass compared to the total volume of blood

  5. Hb: main component of RBCs and transports O2 and CO2

  6. RBC indices: calculated values of size and Hb content of RBCs; important in anemia evaluations

    1. Mean corpuscular volume (MCV): measure of the average volume of an RBC

    2. Mean corpuscular hemoglobin concentration (MCHC): amount of Hb relative to the size of the RBC

    3. Mean corpuscular hemoglobin (MCH): amount of Hb per RBC

  7. Stained red cell examination (film or peripheral blood smear); helps examine cells and its components

  8. Platelet count (often included in CBC): Thrombocytes are necessary for clotting and control of bleeding

  9. RBC distribution width (RDW): indicates degree variability and abnormal cell size

  10. Mean platelet volume (MPV): index of platelet production

These tests are described in detail in the following pages.

Clinical Alert

Hct of 20% can lead to cardiac failure and death.Hct of >60% is associated with spontaneous clotting of blood.Hb value of 5.0 g/dL (50 g/L) leads to hypoxia, ischemic heart disease, and death.Hb value of >20.0 g/dL (>200 g/L) results in hemoconcentration and clogging of capillaries.A critical decrease in platelet value to 20 × 103/mm3 (20 × 109/L) is associated with a tendency for spontaneous bleeding, prolonged bleeding time, petechiae, and ecchymosis

Normal Findings ⬆ ⬇

See Table 2.2.

Interventions ⬆ ⬇

Pretest Patient Care for CBC, and Differential (Diff) Count (All Components)

  1. Explain test procedure. Warn that slight discomfort may be felt when skin is punctured. Refer to procedure for Venipuncture for additional information.

  2. Avoid stress, if possible, because altered physiologic status influences and changes normal hemogram values.

  3. Select CBC components ordered at regular intervals (e.g., daily, every other day). These should be drawn consistently at the same time of day for reasons of accurate comparison; natural body rhythms cause fluctuations in laboratory values at certain times of the day.

  4. Dehydration or overhydration can dramatically alter values; for example, large volumes of IV fluids can “dilute” the blood, and values will appear as lower counts. The presence of either of these states should be communicated to the laboratory.

  5. Fasting is not necessary. However, fat-laden meals may alter some test results because of lipidemia.

  6. Some medications and other substances can alter results. Obtain a current medication history from the patient.

  7. A high WBC count or diseases that cause RBCs to agglutinate may alter test results.

Posttest Patient Care for CBC, and Differential (Diff) Count (All Components)

  1. Apply manual pressure and dressing to the puncture site on removal of the needle.

  2. Monitor the puncture site for oozing. Maintain pressure dressings on the site if necessary. Notify the healthcare provider of unusual problems with bleeding. If a hematoma develops, apply a compress. If the hematoma is large, assess pulses distal to the phlebotomy site.

  3. Have the patient resume normal activities and diet.

  4. Bruising at the puncture site is not uncommon. Signs of inflammation are unusual and should be reported if the inflamed area appears large, if red streaks develop, or if drainage occurs.

  5. Evaluate the outcome and counsel the patient appropriately about anemia, polycythemia, risk for infection, and related blood disorders.

  6. Monitor patients with serious platelet defects for signs and symptoms of gastrointestinal bleeding, hemolysis, hematuria, petechiae, vaginal bleeding, epistaxis, and bleeding from gums.

  7. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Clinical Alert

Never apply a total circumferential dressing and wrap because this may compromise circulation and nerve function if constriction, from whatever cause, occurs

Interfering Factors ⬆

RBC Count

Hct

  • Physiologic variants affect Hct outcomes: age, gender, hydration level, and physiologic hydremia of pregnancy.

Hb

  • Physiologic variations affect test outcomes: high altitude, hydration level, age, pregnancy, and many drugs.

MCHC

  • High values may occur in newborns and infants.

  • Presence of leukemia or cold agglutinins may increase levels. MCHC is falsely elevated with a high blood concentration of heparin.

MCH

  • Hyperlipidemia and high heparin concentrations falsely elevate MCH values.

  • WBC counts greater than 50,000/mm3 falsely elevate Hb values and falsely elevate the MCH.

WBC Count

  • Hourly variation, age, exercise, pain, temperature, and anesthesia affect test results.

Neutrophils and Eosinophils

  • Physiologic conditions such as stress, excitement, exercise, and obstetric labor increase neutrophil levels. Steroid administration affects levels for up to 24 hours.

  • The eosinophil count is lowest in the morning and then rises from noon until after midnight. Do repeat tests at the same time every day. Stressful states such as burns, postoperative states, and obstetric labor decrease the count. Drugs such as steroids, epinephrine, and thyroxine affect eosinophil levels.

Platelets

  • Physiologic factors include high altitudes, strenuous exercise, excitement, and premenstrual and postpartum effects.

  • A partially clotted blood specimen affects the test outcome.