The complete blood count is a commonly ordered lab test that measures the cellular components in the bloodstream. The white blood count is a major component of this test and can supply important information concerning a patient's state of health or disease.
White Blood Cells
White blood cells, also called leukocytes, are constantly being produced in areas of bone marrow scattered throughout the body. These cells are released into the bloodstream when mature, and they circulate for several days or more before being replaced. Their main purpose is to monitor for, and respond to, infection or other foreign invaders attempting to enter the body. Large studies have determined the numbers and proportions of leukocytes normally present in blood, as well as typical changes seen with different disease processes.
The Test
A phlebotomist collects the blood sample, most commonly by needle puncture of an arm or hand vein. Smaller samples may sometimes be collected by capillary tube or with a lancet. The blood is transported to the laboratory in a tube with anticoagulant, where it is analyzed using an automated instrument. If any results are too far outside of expected ranges, a medical technologist may examine the blood directly under a microscope.
Test Results
A complete blood count measures numerous parameters involving red cells, white cells and platelets. According to the Mayo Clinic, the total white cell count is usually in the range of 3,500 to 10,500 cells per mcL. Individual labs may have slightly different normal values, determined by their patient population and instrumentation. Neutrophils make up the majority of white cells in healthy adults, followed by lymphocytes, monocytes, eosinophils and basophils. In young children, lymphocytes are the predominant cell, with neutrophils second.
Leukopenia
A low white cell count, or leukopenia, may reflect problems with bone marrow production or decreased circulating time of cells in the bloodstream. Causes of leukopenia include medications, chemotherapy, chemical exposure, bone marrow failure, autoimmune diseases and some disorders of the liver or spleen. According to the National Cancer Institute, as the neutrophil count drops below 1,000 per mcL, the risk of infection begins to increase.
Leukocytosis
A high white cell count, or leukocytosis, may indicate a reactive or neoplastic process. Elevated counts are most commonly due to infection or inflammation but may also be associated with bleeding, medications, allergies, severe stress, leukemia and lymphoma. With leukemia or lymphoma, counts may reach 100,000 cells per mcL or greater.
Interpretation
Both total white blood count and the mix of cell types present contribute information regarding a patient's health status. All results must be evaluated together with the medical history, physical examination and other studies performed to best aid in reaching a diagnosis.
