Complete Blood Count
Blood analysis is a common lab test used to diagnose and monitor the body's response to diseases. Find out what it can do for you.
Overview Blood carries out many vital functions as it circulates through the body. It transports oxygen from the lungs to other body tissues and carries away carbon dioxide. It carries nutrients from the digestive system to the cells of the body, and carries away wastes for excretion by the kidneys. Blood helps our body fight off infectious agents, stops bleeding through its clotting ability and regulates our body temperature.
Doctors rely on many blood tests to diagnose and monitor diseases. Some tests measure the components of blood itself. Others examine substances found in the blood to identify abnormal function of various organs.
What is a complete blood cell count (CBC)? A CBC is one of the most commonly performed blood tests. It measures the number of red blood cells, white blood cells and platelets.
Platelets are needed for blood to clot. Red blood cells carry oxygen from the lungs to the tissues and take carbon dioxide away. White blood cells help fight infections.
The CBC determines the number of blood cells and platelets, the percentage of each type of white blood cell and the content of hemoglobin (an oxygen-carrying protein in red blood cells). It also usually measures the size and shape of red blood cells.
A CBC will also show a hematocrit level. This tells how many red blood cells there are in relation to the amount of blood that you have.
How is the CBC performed? A blood sample for a CBC is taken from a vein, usually in your forearm. The puncture site is cleaned with an antiseptic. A tourniquet (an elastic band) or blood pressure cuff is then placed around the upper arm to restrict blood flow through the vein. This causes the vein below the tourniquet to fill with blood. A needle is inserted into the vein, and the blood is collected in a vial or syringe. During the procedure, the tourniquet is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.
When a blood test is done on an infant or child, a small amount of blood can sometimes be taken from the finger or earlobe.
What happens after the test? If there is swelling at the puncture site, you can apply ice to the area. Check with your doctor before applying ice if you have diabetes or problems with your circulation.
What are normal results? Normal red blood cell counts vary with your age and gender.
Men: 4.2 to 5.4 million red blood cells per microliter (mL) of blood Women: 3.6 to 5.0 million red blood cells per mL of blood Children: 4.6 to 4.8 million red blood cells per mL of blood
Hemoglobin concentration matches closely with the red blood cell count. Normal white blood cell counts also depend on your age. They can range from 4,000 to 10,000 white blood cells per cubic millimeter of whole blood.
What do abnormal results mean? A low red blood cell count or hemoglobin level indicates anemia, or severe bleeding. An elevated red cell count or hemoglobin level may indicate polycythemia, a rare blood disorder. Abnormally shaped red blood cells can also signal problems:
Sickle-shaped cells indicate sickle cell disease. Small red blood cells may mean an iron deficiency. Large oval red blood cells suggest folic acid or vitamins B12 deficiency (pernicious anemia).
The number of white blood cells may rise or fall significantly in certain diseases. An elevated white blood cell count can indicate an infection. This can include abscess, meningitis, pneumonia, appendicitis or tonsillitis. A high count may also be caused by inflammation, stress, leukemia or by dead tissue from burns, heart attack or gangrene. A low white blood cell count may mean bone marrow problems or certain infectious diseases. A number of disorders can lead to a low platelet count, which raises the risk of bleeding.