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Complete Blood Count (CBC)

What is a complete blood count?

A complete blood count or CBC is a blood test that measures many different parts and features of your blood, including:

  • Red blood cells, which carry oxygen from your lungs to the rest of your body
  • White blood cells, which fight infection. There are five major types of white blood cells. A CBC test measures the total number of white cells in your blood. A test called a CBC with differential also measures the number of each type of these white blood cells
  • Platelets, which help your blood to clot and stop bleeding
  • Haemoglobin, a protein in red blood cells that carries oxygen from your lungs and to the rest of your body
  • Hematocrit, a measurement of how much of your blood is made up of red blood

A complete blood count may also include measurements of chemicals and other substances in your blood. These results can give your health care provider important information about your overall health and risk for certain diseases.

Other names for a complete blood count: CBC, full blood count, blood cell count

What is it used for?

A complete blood count is a commonly performed blood test that is often included as part of a routine checkup. Complete blood counts can be used to help detect a variety of disorders including infections, anaemia, diseases of the immune system, and blood cancers.

Why do I need a complete blood count?

Your health care provider may have ordered a complete blood count as part of your checkup or to monitor your overall health. In addition, the test may be used to:

  • Diagnose a blood disease, infection, immune system and disorder, or other medical conditions
  • Keep track of an existing blood disorder
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What happens during a complete blood count?

A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

Will I need to do anything to prepare for the test?

You don’t need any special preparations for a complete blood count. If your health care provider has also ordered other blood tests, you may need to fast (not eat or drink) for several hours before the test. Your health care provider will let you know if there are any special instructions to follow.

Are there any risks to the test?

There is very little risk of having a blood test. You may have slight pain or bruise at the spot where the needle was put in, but most symptoms go away quickly.

What do the results mean?

A CBC counts the cells and measures the levels of different substances in your blood. There are many reasons your levels may fall outside the normal range. For instance:

  • Abnormal red blood cell, haemoglobin, or hematocrit levels may indicate anaemia, iron deficiency, or heart disease
  • Low white cell count may indicate an autoimmune disorder, bone marrow disorder, or cancer
  • High white cell count may indicate an infection or reaction to the medication

If any of your levels are abnormal, it does not necessarily indicate a medical problem needing treatment. Diet, activity level, medications, a women’s menstrual cycle, and other considerations can affect the results. Talk to your health care provider to learn what your results mean.

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Is there anything else I need to know about a complete blood count?

A complete blood count is only one tool your health care provider uses to learn about your health. Your medical history, symptoms, and other factors will be considered before a diagnosis. Additional testing and follow-up care may also be recommended.