Complete Blood Count
What is a complete blood count (CBC)?
A complete blood count (CBC) is a blood test. It helps healthcare providers detect a range of disorders and conditions. It also checks your blood for signs of medication side effects. Providers use this test to screen for diseases and adjust treatments.
A CBC measures and counts your blood cells. Your provider takes a sample of your blood and sends it to a lab. The lab does a series of tests to evaluate your blood cells. These tests help your provider monitor your health.
When is a CBC performed?
You may need a CBC if you have symptoms such as:
- Bruising or bleeding.
- Fatigue, dizziness or weakness.
- Fever, nausea and vomiting.
- Inflammation (swelling and irritation) anywhere in the body.
- Joint pain.
- Problems with heart rate or blood pressure.
Why do healthcare providers order CBCs?
CBCs are an important part of a yearly physical exam. Providers also order CBCs to monitor the side effects of some prescription medications.
Your provider may order a CBC to:
- Detect abnormalities in your blood that may be signs of disease.
- Diagnose or monitor many different disorders, conditions and infections.
- Evaluate your overall health.
- Rule out conditions, disorders and disease.
- Monitor various blood diseases.
What does a CBC look for?
A CBC does many tests to measure and study red blood cells, white blood cells and platelets. Red blood cells carry oxygen throughout the body. White blood cells are part of your immune system. They help your body fight infection. Platelets help your body clot.
A CBC measures, counts, evaluates and studies many aspects of your blood:
- CBC without differential counts the total number of white blood cells.
- CBC with differential. There are five kinds of white blood cells. The differential looks at how many of each kind of white blood cell you have.
- Hemoglobin tests measure hemoglobin, the protein in red blood cells that carries oxygen.
- Hematocrit describes the concentration of red blood cells in your blood.
A CBC tells your provider:
- How many new blood cells your body is creating.
- Number of red blood cells (RBC or erythrocytes), white blood cells (WBC or leukocytes) and platelets.
- Size and shape of blood cells.
What does a CBC detect?
A CBC blood test can help your provider diagnose a wide range of conditions, disorders, diseases and infections, including:
- Anemia (when there aren’t enough red blood cells to carry oxygen through the body).
- Bone marrow disorders, such as myelodysplastic syndromes.
- Disorders such as agranulocytosis and thalassemias and sickle cell anemia.
- Infections or other problems that cause abnormally low white blood cell count or high white blood cell count.
- Several types of cancer, including leukemia and lymphoma.
- Side effects of chemotherapy and some prescription medications.
- Vitamin and mineral deficiencies.
What should I expect during a complete blood count (CBC)?
You don’t need to do anything to prepare for a CBC. Your provider cleans your arm and inserts a needle. The needle may sting or pinch a little, but it shouldn’t hurt. In infants, providers usually insert the needle into the baby’s heel.
Through the needle, your provider removes a sample of your blood and collects it in a tube. Sometimes, your provider takes more than one tube of blood.
After drawing blood, your provider removes the needle and places a bandage on your arm. Your provider sends the blood to a lab. Your body quickly rebuilds its blood supply.
What should I expect after the test?
You’ll have some gauze and a bandage on your arm, secured with tape. Your arm may be a little sore for a few hours. You may develop a small bruise where your provider inserted the needle.
What are the benefits of this test?
A CBC gives your provider a picture of your overall health. Using a small amount of blood, a CBC can help detect hundreds of conditions, disorders and infections. It allows your provider to monitor your health, screen for disease and plan and adjust treatment.
What are the risks of this test?
A CBC is a safe, common test. There are no risks involved, and your provider only removes a small amount of blood. Rarely, some people feel a little faint or lightheaded after a CBC.
Results and Follow-Up
When should I know the results of the test?
Results are usually ready within a few days. Sometimes it only takes 24 hours to get results. Your provider will contact you to explain the results and discuss next steps. If your blood cell counts are outside of the normal range, your provider may order follow-up tests.
When should I call my doctor?
Your provider will review the results of your CBC with you. If you have questions about the results, call your provider.
A note from Cleveland Clinic
Healthcare providers use complete blood counts to manage disease and help you stay healthy. With one sample of blood, CBCs can help screen for hundreds of disorders, conditions and infections. A CBC can detect conditions early, sometimes before you have symptoms, so treatment can start as soon as possible. CBCs are an essential tool in maintaining good overall health.