Overview

A complete blood count (CBC) is a blood test. Healthcare professionals use it to look at overall health and to find a wide range of conditions, including anemia, infection and leukemia.

A complete blood count test measures the following:

  • Red blood cells, which carry oxygen.
  • Hemoglobin, the oxygen-carrying protein in red blood cells.
  • Hematocrit, the amount of red blood cells in the blood.
  • White blood cells, which fight infection.
  • Platelets, which help blood to clot.

A complete blood count can show when cell counts are higher or lower than the standard results. Results that are not standard may point to a medical condition that calls for more testing.

Why it's done

A complete blood count is a common blood test done for many reasons, including:

  • To look at overall health. A complete blood count can be part of a medical exam to check health. It also can look for conditions such as anemia or leukemia.
  • To diagnose a medical condition. A complete blood count can help find the cause of symptoms such as weakness, tiredness and fever. It also can help find the cause of swelling and pain, bruising, or bleeding.
  • To check on a medical condition. A complete blood count can help check on conditions that affect blood cell counts.
  • To check on medical treatment. A complete blood count may be used to check blood cell counts after treatments with radiation or certain medicines.

How you prepare

Before a complete blood count test, you can eat and drink as usual. If you also have other blood tests, you may need to fast for a time before the test. Ask your healthcare professional what you need to do.

What you can expect

For a complete blood count, a member of the health care team takes a sample of blood by putting a needle into a vein in your arm, usually at the bend in your elbow. The blood sample is sent to a lab. After the test, you can return to your usual activities right away.

Results

The following complete blood count results are standard results for adults. Blood most often is measured in cells per liter (cells/L) or grams per deciliter (g/dL).

Red blood cell count

Male: 4.35 trillion to 5.65 trillion cells/L

Female: 3.92 trillion to 5.13 trillion cells/L
Hemoglobin

Male: 13.2 to 16.6 g/dL

Female: 11.6 to 15 g/dL
Hematocrit

Male: 38.3% to 48.6%

Female: 35.5% to 44.9%
White blood cell count

3.4 billion to 9.6 billion cells/L
Platelet count

Male: 135 billion to 317 billion/L

Female: 157 billion to 371 billion/L

Not a complete diagnostic test

A complete blood count (CBC) may not give all the answers about a diagnosis. Results outside the standard range may need follow-up. To decide, a healthcare professional may look at the results of other tests, the CBC results and the person's overall health.

For instance, for someone who has no symptoms of illness, CBC results slightly outside the standard range may not need follow-up. But for someone having cancer treatment, CBC results outside the standard range might mean a need to change the treatment.

When results are way above or below the standard ranges, a healthcare professional may ask you to see a doctor who treats blood conditions. This type of doctor is called a hematologist.

What the results may show

Results above or below the standard ranges on a complete blood count might point to an issue.

  • Red blood cell count, hemoglobin and hematocrit. These three each measure a feature of red blood cells.

    Lower than usual measures in these areas are a sign of anemia. Anemia has many causes. They include low levels of certain vitamins or iron, blood loss or another medical condition.

    A red blood cell count that's higher than usual is called erythrocytosis. A high red blood cell count or high hemoglobin or hematocrit levels could point to a blood cancer or heart disease.

  • White blood cell count. A low white blood cell count is called leukopenia. An autoimmune condition that destroys white blood cells may cause it. Other causes include bone marrow issues or cancer. Certain medicines also can cause low white blood cell counts.

    A white blood cell count that's higher than usual most often is due to an infection or swelling and irritation, called inflammation. Or it could point to condition of the immune system or the bone marrow. A high white blood cell count also can be a sign of certain cancers or the result of medicines or hard exercise.

  • Platelet count. A platelet count that's lower than usual is called thrombocytopenia. If it's higher than usual, it's called thrombocytosis. Either can be a sign of a medical condition or a side effect from medicine. A platelet count that's outside the standard range likely needs more tests to diagnose the cause.

Your healthcare professional can tell you what your complete blood count results mean.

By Mayo Clinic Staff