The blood circulating in the arteries and veins of the body contains two major components: plasma, the liquid fraction of blood; and the cellular fraction containing erythrocytes (red blood cells) and many other cell types which are suspended in the plasma. Performing a complete blood count, or CBC, on the cellular components of the blood can give a physician important insights into what may be causing illness in an individual.
The Erythrocyte Count and the Hematocrit
Erythrocytes, or red blood cells (RBCs), are the oxygen carrying cells of the circulation. RBCs make up approximately 40 to 50% of the total blood volume. Interestingly, the mature RBCs found in mammals have no nucleus, while those in birds do. They develop from a bone marrow derived cell lineage tracing from erythroblasts, which lead to reticulocytes which then form RBCs, and for mammals extruding the cell nucleus along the way. A typical normal number of RBCs found in a blood specimen can range from four to six million cells per microliter (one one thousandth of a milliliter) of blood depending upon gender and the altitude at which the person lives.
The volume that the red blood cells occupy in the blood sample is measured and represents a value known as the hematocrit. This value depends not only on the absolute number of RBCs in a given blood sample but also how big the RBCs are. Changes in the RBC count or hematocrit can indicate conditions such as anemia if the values are low or dehydration if the values are higher than normal. Many other insights can be gained from these values. Additional measurements include the amount of hemoglobin and characterization of the size and shape of the erythrocytes.
The Leukocyte Count
White blood cells (WBCs), also known as leukocytes, are the infection fighting cells of the blood. There are several different kinds of leukocytes in the blood including lymphocytes, basophils, eosinophils, neutrophils and monocytes. These different classes are identified and counted in another type of test which identifies these different white blood cells based on characteristics of their appearance (the differential count). Normal ranges for total WBC counts can range from 4500 to 10,000 per microliter. WBC numbers can change as a result of infections (usually increased), drug treatments (higher or lower depending upon the drug), exposure to radiation (decreased) and many other factors. Higher than normal WBC counts are referred to as leukocytosis, while lower than normal WBC counts are referred to as leukopenia.
The Platelet Count
Platelets, otherwise known as thrombocytes, are critically involved in the ability of blood to form a clot after injury to a blood vessel wall has caused bleeding. They are actually specific fragments of cells generated in the bone marrow. The number of platelets typically seen in a blood sample range from 150,000 to 400,000 per microliter. When platelet counts are below the normal range a person can have trouble clotting their blood quickly enough. This is known as thrombocytopenia. When platelet counts are high this is referred to as thrombocytosis.
When all of the cell values are considered together they can provide any of a number of insights into what may be causing an illness in a patient or how someone is responding to certain types of treatments. It is a low cost, relatively simple and longstanding reliable tool in the armamentarium of the medical practitioner.
To learn more about the complete blood count visit the blood count pages at MedLine Plus at the US National Library of Medicine.
Kenneth Rosen - I am a medical research doctor with more than 20 years of experience studying how the nervous system and skeletal muscle develop and ...
Join the Conversation