The complete blood count (CBC) with differential is one of the most common laboratory tests performed today. It gives information about the production of all blood cells and identifies the patient’s oxygen-carrying capacity through the evaluation of red blood cell (RBC) indices, hemoglobin, and hematocrit.
It also provides information about the immune system through the evaluation of the white blood cell (WBC) count with differential. These tests are helpful in diagnosing anemia, certain cancers, infection, acute hemorrhagic states, allergies, and blood dyscrasias.
Nurses are frequently challenged to obtain and evaluate all or parts of the CBC as a part of the patient’s management. An enhanced understanding of this laboratory test is essential to providing quality nursing care.
The components of the complete blood count include hemoglobin, hematocrit, RBC indices, WBC with differential and platelets.
Red Blood Cells
RBC’s, also known as erythrocytes, are produced in the bone marrow and then remain in the blood stream for about 120 days before dying off. Normal RBC range in males is 4.7 to 6.1 million cells per microliter (cells/mcL) and females 4.2 to 5.4 million cells/mcL. They are highly specialized for oxygen (02) transport they contain hemoglobin which has a high affinity for oxygen.
Specific characteristics of RBC’s are called RBC indices which help to determine what type of anemia a patient may have. The mean corpuscular volume (MCV) tells you the size of an RBC. There are 3 size descriptions:
- Normocytic, normal cell size, as determined by a MCV of 75-94fl*;
- Macrocytic, large cell size, as indicated by MVC of > 100fl which can point to folate and B12 deficiency or aplastic anemia; and
- Microcytic indicated by a MCV of < 75fl possibly indicating lead poisoning and iron deficiency anemia.
Generally, small size RBC’s result in lower MCV, while larger RBC’s result in higher MCV’s.
The mean corpuscular hemoglobin concentration (MCHC) is a measurement of the average amount of Hgb in RBC’s compared to the average size of the RBC’s. When the MCHC level are normal the RBC’s are called normochromic; when the values are low the RBC’s are considered hypochromic and are a pale color and when the MCHC is high they are considered hyperchromic or bright color.
The mean corpuscular hemoglobin (MCH) reflects the average amount of hemoglobin is the patient’s RBC’s. Normal values are 27 – 31 picograms/cell. Alterations in the MCH levels are related to the mean corpuscular hemoglobin concentration.
Hemoglobin & Hematocrit
Closely connected to the RBC indices are the Hemoglobin and Hematocrit values.
Hemoglobin is responsible for the red color of the RBC’s it contains a heme portion which is the iron -containing part that combines with oxygen in the lungs and releases it in the tissues.
The globin portion works with the heme portion to prevents the iron from being oxidized by the oxygen (O2) it is carrying. Blood values range from 12.1 to 15.1 gm/dl in women and 13.8 t0 17.2 gm/dl in men.
Abnormal values that are lower than normal may be due to anemia, bleeding and over hydration with higher than normal values due to hypoxia, dehydration and pulmonary fibrosis.
The hematocrit (Hct) is the percentage of total volume of RBC’s in the blood sample. It parallels the Hgb levels and is three times the Hgb level in patients and when alterations occur in one value it will affect the second value. Normal values in men and women are 14-18g/dl and 12-16g/dl in women, respectively.
Clinical causes of an elevated Hct level can be a compensatory mechanism in CHF, COPD as well as those patients who are dehydrated or live at high altitudes
White Blood Count
The white blood cell count (WBC) with differential breaks down the WBC’s into their 6 subtypes. They are expressed as a percentage (%) of the total WBC count and the sum will equal 100%. They are broken down into either granulocytes which have granules in their cytoplasm, or agranulocytes which do not have granules in their cytoplasm.
Neutrophils are granulocytes which are the most abundant cells in the WBC count equaling 31-51% of the total WBC’s in circulation. Their role is to be the first line of defense against acute infection. Immature neutrophils called bands, or rods, are released when there is increased demand for neutrophils such as in bacterial infections.
Basophils are the granulocytes that are associated with systemic allergic reactions, chronic inflammatory conditions and hypersensitivity reactions. They represent 0-1% of the total WBC differential.
Finally, Eosinophils which are involved in the protection of the body against parasitic infections (parasitic worms) and also in the phagocytosis of allergens and proteins associated with inflammatory reactions. Increased numbers of eosinophils is called eosinophilia, the clinical implications are in hay fever, drug reaction and asthma. Eosinopenia, decreased eosinophils, occurred in corticosteroid therapy, stress and shock.
Agranular leukocytes (agranulocytes) include lymphocytes and monocytes which do not contain granules. Lymphocytes account for 25-35% of all WBC’s and are the 2nd most abundant type of blood cells and are involved in the adaptive immune response of the body. There are two types: T cells and B cells.
T cells are processed in the thymus gland and are responsible for cell-mediated immunity which involves phagocytosis, cytotoxic T cells and the release of cytokines in order to mount a defense against antigens. However, B cells are responsible the formations of immunoglobulins ( Humoral immunity) which occurs when an antigen (foreign body) is presented to the B cell which then releases antibodies specific for a specific antigen.
Monocytes are the largest of the white blood cells and accounted for 3-7% of the blood differential. Monocytes will migrate via the blood stream to other tissues where they will reside and then differentiate into macrophages. They will have different names depending upon where they are found, for example, Kupffer cells are macrophages that are found in the liver.
When pathogens enter the body monocytes move towards the specific pathogenic cell and eventually adhere to it and engulf it and carry out the process of phagocytosis. An increase in monocytes (Monocytosis) can be due to chronic inflammatory disease, viral infection and Leukemia. A decrease in monocytes (Monocytopenia) is seen in bone marrow suppression or failure.
Platelets
Platelets, also known as thrombocytes, are the smallest of the cells found in the blood stream and play a vital role in the process of blood clotting with normal values ranging from 150,000 to 300,000 mm3.
Platelets help to control bleeding by two means (1) when a blood vessel in the body is damaged, platelets aggregate or clump at the site of the injury to form a clot to seal the injured site (2) there is the stimulation of clotting factors (coagulation cascade) which are released to form fibrin strands which wind around the platelets to strengthen the clot.
Increases in platelet count or thrombocytosis are asymptomatic until the blood values get to > than 1,000,000/L when inappropriate clotting may occur. Thrombocytosis may be due to rare conditions that lead to an overproduction of platelets. Thrombocytopenia or decreased platelet count or values < 150,000 / L can be due to destruction of the platelets or decreased production by the bone marrow. This can occur in Immune thrombocytopenic purpura (ITP) and Drug-induced thrombocytopenia
Conclusion
The complete blood count is a basic screening test, however, by having the knowledge to correctly interpret the results on the CBC, nurses can be instrumental in ascertaining the severity of patient conditions such as hemorrhage, anemia and infection; with the ultimate goal of providing your patients the finest nursing care possible.
References for this article can be accessed here.
*Note: normal range values may vary slightly among different laboratories
Donna W. Driscoll is associate professor of nursing and coordinator of the pediatric nurse practitioner track at Molloy College, Rockville Centre, N.Y.
