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Diagnostic Tests

Blood Test: Complete Blood Count

What’s a Complete Blood Count?

Doctors may want you to get a type of medical test called a complete blood count (CBC) if you’re feeling more tired than usual, seem to have an infection, or have unexplained bruising or bleeding.

The CBC is a common blood test that helps doctors get information about the three major types of cells in the blood:

  1. Red blood cells. These are the cells that carry oxygen from the lungs to the rest of the body. The CBC measures the number of red blood cells in the body, along with other information about red blood cells, such as amounts of an oxygen-carrying protein called hemoglobin. This information can help doctors test a person for anemia, which can happen due to loss of blood, or problems with the production or destruction of red blood cells.
  2. White blood cells. These cells help the body fight infection. The CBC measures the number of white blood cells (also called leukocytes) in the blood and also looks at the different types of white blood cells in the blood and how they relate to one another. Problems with the number of white blood cells can be signs that a person has an infection or other stress in the body. For example, a bacterial infection can cause the white blood cell count to go up or down. Medications, like certain anticancer drugs, also can affect a person’s white blood cell count.
  3. Platelets. These cells play an important role in blood clotting and the prevention of bleeding. When a blood vessel is damaged or cut, platelets clump together and plug the hole until the blood clots. If the platelet count is too low, a person can be in danger of bleeding in any part of the body.


You don’t need to do anything special to get ready for a complete blood count test. However, you should tell your doctor about any medications (including over-the-counter medicines or herbal supplements) you’re taking because certain drugs might alter the test results.

It can help to wear a T-shirt or other short-sleeved top on the day of the test to make things faster and easier for the technician who will be drawing the blood.

The Procedure

A health professional usually draws the blood from a vein in your arm — most often on the inside of the elbow, but sometimes on the back of the hand. The technician cleans the skin surface with antiseptic and ties an elastic band (tourniquet) around the upper arm so the veins swell with blood and are easy to see.

Next, it’s time for the needle. It should feel like a quick pinprick. Occasionally, it can be hard to find a vein so a nurse, doctor, or technician may need to try more than once. That’s not the norm, though — most people’s veins are easy to find.

It’s best to try to relax and stay still during the procedure since tensing muscles can make it harder and more painful to draw blood. And if you don’t want to watch the needle being inserted or see the blood collecting, you don’t have to. Look the other way and maybe relax by focusing on saying the alphabet backwards, doing some breathing exercises, thinking of a place that makes you happy, or listening to your favorite music.

The technician will draw the blood so it collects in a vial or syringe. Collecting blood will only take a few minutes. Once the technician has enough blood, he or she removes the needle and covers the area with cotton or a bandage to stop the bleeding. After the test, you may notice some bruising — that’s normal and it should go away in a few days. Don’t be afraid to ask the technician if you have any questions about the blood draw.


A blood test is a safe procedure and there are no real risks. Some people may feel faint or lightheaded during a blood test. And, while nobody really loves needles, a few teens have a strong fear of them. If that’s you, talk to your doctor since there ways to make the procedure easier for you.


The blood sample is processed by a machine. It usually only takes a few hours or a day or so for your doctor to get the results (in an emergency, results on some blood tests can come back much faster). If the results point to anemia, infection, or other problems, your doctor may want to do other tests to find out what the cause is and how to treat it.

Reviewed by: Yamini Durani, MD
Date reviewed: August 2014