Understanding Clinical Lab Tests: CBC and Blood Cell Analysis
Interpretation of Clinical Laboratory Tests
Common blood tests are performed to identify the general health status of a patient and to determine the positive or negative effects of therapy. These tests are important for respiratory therapists because they may influence and reflect the patient’s lung functions.
CBC: Complete Blood Count
Venous blood sample. Examines the formed elements in the blood:
- White Blood Cells (WBCs/Leukocytes) – Segmented neutrophils, Bands, Eosinophils, lymphocytes, monocytes.
- Red Blood Cells (RBCs/Erythrocytes) – Size, number, and presence of hemoglobin.
- Platelets (Thrombocytes) – Number present.
White Blood Cells (WBCs)
- Leukocytosis – Elevation of WBCs due to stress, infection, or trauma.
- Leukopenia – Decreased number of WBCs, less common than leukocytosis. Occurs when the immune system is overwhelmed by infection, therapy suppression, or disease. Common in elderly patients with severe pneumonia, diseases of the bone marrow (e.g., leukemia, lymphoma), and patients undergoing chemotherapy and radiation therapy.
Differential of the WBCs
Determination of the exact number of each type of WBC present. Most circulating WBCs are either neutrophils or lymphocytes. A significant elevation of WBCs (more than 15,000/mm3) will occur only when either neutrophils or lymphocytes are responding to an abnormality. Basophils, eosinophils, and monocytes make up such a small proportion of the WBCs that they are not likely to cause a major increase in the WBC count when responding to a disease.
Neutrophils
- Neutrophilia – Elevation of the absolute value of neutrophils.
- Bands – Immature neutrophils, the majority of which are located in the bone marrow.
- Segs – Mature neutrophils.
Severe infection causes the bone marrow to release stores of any available neutrophils (bands or segs). If bands or segs are elevated in the CBC, the patient is most likely experiencing a severe bacterial infection. Segs are also stored in clusters along the walls of some blood vessels. These neutrophils can be recalled into the circulating pool under certain conditions. The release of catecholamines with acute stress is a common example of marginated neutrophils being recalled into the circulating blood. In such conditions, the neutrophilia will be primarily made up of segs and few bands. This form of neutrophilia is transient.
Neutropenia
Reduced number of neutrophils. Not as common as neutrophilia but does occur in patients with bone marrow disease (e.g., lymphoma, leukemia, etc.), patients undergoing cancer treatments, those with autoimmune disorders, and HIV-infected patients. Neutropenia puts the patient at risk for the development of infection.
Red Blood Cell Count
Reflects the ability of the red blood cells to carry oxygen.
- Anemia – Low RBC count, suggests either inadequate production of RBCs by the red bone marrow or excessive loss of blood. Decreased oxygen-carrying capacity. Patients most likely have hypoxia.
- Polycythemia – Elevated RBC count suggests that the bone marrow is being stimulated to produce extra RBCs in response to chronically low blood oxygen levels. Patients who live at high altitude and those with chronic lung disease are most likely to experience chronic hypoxia and to develop polycythemia.