Blood is a fluid body tissue. It is comprised of red blood cells (rbc), white blood cells, and platelets. Red blood cells contain the protein hemoglobin, responsible for the color of blood and carrying oxygen to tissues. White blood cells participate in body defense. While there are several types of white blood cells, the two most abundant types are usually the neutrophils and the lymphocytes. Platelets are cell fragments which help control bleeding.
A CBC is a simple blood test that analyzes red blood cells, white blood cells, and platelets. The test also measures the concentration of hemoglobin and the hematocrit, also known as hct or crit. The hematocrit is defined as the percentage of blood composed by rbcs. As a rule of thumb, the hemoglobin value is approximately one-third the hematocrit value.
A routine CBC will determine the white blood cell count, but it does not provide information about the types of wbcs present. This information can be obtained by performing a differential wbc count. Differentials are best performed by using a microscope to examine a stained specimen of blood. Blood smears are also used to evaluate the size and shape of blood cells. Many conditions (e.g., sickle cell anemia) can be diagnosed by the microscopic appearance of blood.
The hematocrit may be performed using automated lab equipment, but it may also be done manually by obtaining a small sample of blood in a tiny hollow tube and spinning it with a centrifuge.
The results of a CBC are compared to standardized normal values for a specific lab. There is a range of values that is considered normal for each component of a CBC.
It only takes a small sample of blood to perform a CBC; in some cases, sometimes only a drop of blood is needed. The blood can be collected in several ways, either by pricking a fingertip with a sharp blade (or doing a “heel stick” in babies) or by inserting a sterile, hollow needle into the vein (usually in the arm) and drawing blood into a test tube. Before collecting the blood, a health care professional swabs the area with alcohol to clean the area and kill any bacteria. A sterile needle or blade is then used to draw blood from the vein or to make a small cut from which blood can be drawn.
Usually the collection of blood with either the finger-prick method or the use of a needle is only slightly painful. Patients may feel a sharp sensation when the needle or blade breaks through the skin. Though uncommon, some patients may experience slight bruising or stinging for a short period of time after the test.
Blood is collected into tubes containing an anticoagulant that keeps blood from clotting. Once the blood has been collected, a machine is used to process and analyze the blood. Abnormal blood counts may be classified by several terms including anemia, the presence of abnormally low rbcs; leukopenia, the presence of a reduced wbc count; and thrombocytopenia, the presence of a reduced platelet count. In contrast, leukemia indicates an abnormally high wbc count due to cancer, and thrombocythemia signifies an elevated platelet count. Once an abnormality is identified, the physician must then determine the underlying cause.
For more information on blood tests in general, click here.
Last updated: 23-Sep-04