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 and inactivates toxins, 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 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. In addition to determining 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), the CBC usually assesses the size and shape of red blood cells.

How is the test performed?

A test to measure red blood cells (hemoglobin or hematocrit) can be done by a nurse, medical technician or phlebotomist who cleans your fingertip and then makes a simple prick with a small lancet. A blood sample is drawn into a tube.

A blood sample for a CBC is taken from a vein, usually in your forearm. The puncture site is cleaned with an antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is 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 be taken from the finger or earlobe.

What happens after the test?

If swelling develops at the puncture site, ice can be applied to the area and later heat can used to ease discomfort.

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 of blood

Women: 3.6 to 5.0 million red blood cells per microliter of blood

Children: 4.6 to 4.8 million red blood cells per microliter of blood

Hemoglobin concentration correlates closely with the red blood cell count. Normal white blood cell counts 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 or hemoglobin count indicates anemia, or severe bleeding. An elevated red cell or hemoglobin count may indicate polycythemia, a rare blood disorder. Abnormally shaped red blood cells can also signal problems: sickle-shaped cells are characteristic of sickle cell disease, small red blood cells may indicate iron deficiency, and large oval red blood cells suggest folic acid or vitamins B12 deficiency (pernicious anemia).

The number of white blood cells may increase or decrease significantly in certain diseases. An elevated white blood cell count can indicate infection, such as an 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 that increases the risk of bleeding.