Pearls of Knowledge © BrainyNurses.com
The CBC with differential is a frequently ordered lab test and provides much information regarding infection, anemia, coagulation, and hydration. When bone marrow suppression occurs, low levels of red blood cells, white blood cells, and platelets will be seen.
Anemia is seen frequently in many different types of patients. When the red blood cells are low, pallor will be seen. If the red blood cells are too numerous, such as in polycythemia vera, the blood is “thicker” and there is a risk for blood clotting and myocardial dysfunction due to the strain.
Men have a higher red blood cell count mostly due to hormonal differences which stimulate higher hemoglobin production and greater muscle mass. For ease in clinical practice, it is helpful to remember the normal level for hemoglobin for everyone is 12-18 g/dL. Using the memory hint below, the normal RBC and hematocrit levels are easy to remember.
A critical hemoglobin level is less than 5.0 g/dL. When the level is < 7.0 g/dL, cardiac output increases to compensate, and a bounding pulse is present. As cardiac output falls, a weak pulse will be present, the pulse pressure will narrow, and signs of shock will develop. When anemia is present, the bone marrow releases reticulocytes, which then mature in the blood or spleen with the help of folic acid and B12. A high reticulocyte count can be seen in several conditions, including acute or chronic blood loss and any pathology causing hemolysis of the red blood cells.
Anemia can be caused by increased blood loss, decreased production due to nutritional deficits or bone marrow suppression, or increased destruction due to genetically abnormal hemoglobin or antibodies. With hemolysis of the red blood cells, the bilirubin level will increase (> 1.2 mg/dL) and the spleen will enlarge.
The red blood cell indices reported on the CBC indicates how the cells are formed and helps to detect the type of anemia. Morphology of the cells is also evaluated to determine types, such as sickle cell.
Mean corpuscular volume (MCV)
Red blood cell distribution width (RDW)
Mean corpuscular hemoglobin concentration (MCHC)
Sickle cell (hemolytic)
Erythroblastosis fetalis (hemolytic)
Platelet are produced in the bone marrow and is the first part of the coagulation cascade. Thrombocytopenia results when too few platelets are produced, or the platelets are destroyed. The lower the value, the more dangerous for the patient.
Based on the Memory Hint, hematocrit should be three times the hemoglobin. In anemia both values will be low. Hematocrit is used to determine hydration status in conjunction with BUN, creatinine, sodium, and specific gravity. Look at these three clinical correlations to determine the hydration status.
Clinical correlation #1
A patient is admitted with hyperosmolar hyperglycemic syndrome. His hemoglobin is 12 g/dL and the hematocrit is 47%.
The hematocrit is elevated in relation to the hemoglobin, signifying dehydration.
Clinical correlation #2
A patient is 4 days post-op from a bowel resection and his IV is running at 125 mL/hr. His hemoglobin is 12 g/dL and the hematocrit is 27%.
The hematocrit is low in relation to the hemoglobin, signifying over hydration.
Clinical correlation #3
A patient with gastroenteritis presents with vomiting and diarrhea for the past three days. His hemoglobin is 10 g/dL and the hematocrit is 33%.
The hemoglobin is low, signifying anemia. The hematocrit is high in relation to the hemoglobin, signifying dehydration.
So, the next time you look at a CBC, consider the following: