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What is Complete Blood Count (CBC)?

The most common blood test, a Complete Blood Count (CBC), is done to get information on the number of red, white and platelet cells in the blood. It tells your medical team the state of your health.

  • The red blood cell (RBC) count provides information about hemoglobin (the oxygen-carrying protein in RBCs) hematocrit (the amount of space RBC’s take up), reticulocyte (how many young RBCs are there), and the Mean Corpuscular Volume (MCV) (average size of RBCs). It shows how well oxygen is traveling from the lungs to the rest of your body.
  • The white blood cell (WBC) count provides information about leukocytes and other WBCs in the blood. WBCs help the body fight infection. An abnormal WBC count may mean there is an infection, inflammation, or other stress in the body.
  • The platelet count shows if person is in danger of bleeding too much. Platelets are the smallest blood cells, and they support blood clotting to stop bleeding. When a blood vessel is damaged or cut, platelets plug the hole by clumping together. If the platelet count is too low, there is a high risk of bleeding somewhere in the body.

What to Expect

This blood test can be done at any time in a lab, hospital or doctor’s office.

A tourniquet or rubber strap is tied around the upper arm to gently keep blood in the vein. Then, a needle with an attached container is inserted into a vein. Often, a large vein in the bend of the elbow or the top of the hand is used. After the container is filled, the needle is removed and a bandage is placed to stop the bleeding.

In newborn infants and small children, veins may be hard to find. A finger prick may be all that’s needed to collect a blood sample.

The entire test takes less than five minutes and is not too uncomfortable. Some people see a slight bruise at the puncture site for a day or two after this test.

More Information

  • When will I learn the test results, and will you explain them to me?
  • What other tests will I need for diagnosis or treatment?
  • If a problem is found, what are my next-steps?