Complete Blood Count

1 What is a Complete Blood Count (CBC)?

A complete blood count (CBC) is a blood test that is utilized to make an evaluation of your overall health and detect a wide range of health disorders, including leukemia, anemia and infection.

A complete blood count test takes a measurement of several components and features of your blood including the following:

  • Red blood cells, which are responsible for transporting oxygen. White blood cells, whose main function is to fight against infection.
  • Hemoglobin, which is the oxygen carrying protein in red blood cells.
  • Hematocrit, the ratio of red blood cells to the fluid component or plasma in your body.
  • Platelets, which help with blood clotting.

Any abnormal increases or decreases in a complete blood count may be an indication of an underlying medical condition that calls for further attention.

2 Reasons for Procedure

A complete blood count is a test that may be carried out for the following reasons:

  • To review your overall health
    Your doctor can recommend a complete blood count as part of a routine medical examination to monitor to general health and also to screen for various disorders, such as anemia or leukemia.
  • To make a diagnosis of a medical condition

    Your doctor may make a suggestion of a complete blood count if you are experiencing weakness, fatigue, fever, inflammation, bruising or bleeding. A complete blood count may help diagnose the cause of the signs and symptoms. If your doctor suspects you have an infection, the test can also help confirm that diagnosis.

  • To monitor a medical condition
    If you have been diagnosed with a blood disorder that affects blood cell counts, your doctor may use complete blood counts to monitor your condition.
  • To monitor medical treatment 

    This test can also be used to monitor your health if you are taking medications that may affect blood cell counts.

3 Potential Risks

A complete blood count is generally a safe procedure and carries no potential risks.

A blood sample will be taken by inserting a needle into your arm.

You can leave immediately after completion.

4 Preparing for your Procedure

Required preparation for your complete blood count includes:

If your blood sample is being solely tested for a complete blood count, you can eat or drink normally before the test.

If your blood sample will be used for any additional tests, you may need to starve for a particular period of time before the test.

Your doctor will give you specific instructions which must be strictly followed.

5 What to Expect

Here you can find out what to expect from your complete blood count procedure.

For a complete blood count, a member of your health care team takes a blood sample by inserting a needle into a vein in your arm, usually at the bend of your elbow.

The blood sample is then sent to the lab for analysis. You can return to your usual activities immediately after the procedure.

6 Procedure Results

The following are the standard complete blood count results for adults:

  • Red blood cell count
    • Male: 4.32-5.72 trillion cells/L* (4.32-5.72 million cells/mcL**)
    • Female: 3.90-5.03 trillion cells/L (3.90-5.03 million cells/mcL)
  • Hemoglobin
    • Male: 13.5-17.5 grams/dL*** (135-175 grams/L)
    • Female: 12.0-15.5 grams/dL (120-155 grams/L)
  • Hematocrit
    • Male: 38.8-50.0 percent
    • Female: 34.9-44.5 percent
  • White blood cell count: 3.5-10.5 billion cells/L (3,500 to 10,500 cells/mcL)
  • Platelet count: 150-450 billion/L (150,000 to 450,000/mcL**)

where * L = liter ** mcL = microliter *** dL = deciliter.

Not a definitive test

A complete blood count is typically not a definitive diagnostic test.

Depending on the reason your doctor recommended this test, results outside the normal range may or may not require follow-up. Your doctor may need to look at the results of a CBC along with results of other blood tests, or additional tests may be necessary. For example, if you're otherwise healthy and have no signs or symptoms of illness, results slightly outside the normal range on a complete blood count may not be a cause for concern, and follow-up may not be needed.

However, if you're undergoing cancer treatment, results of a complete blood count outside the normal range may indicate a need to alter your treatment plan.

In some cases, if your results are significantly above or below the normal ranges, your doctor may refer you to a doctor who specializes in blood disorders (hematologist).

What the results may indicate

Results in the following areas above or below the normal ranges on a complete blood count may indicate a problem.

  • Red blood cell counts, hemoglobin, and hematocrit: The results of your red blood cell count, hemoglobin and hematocrit are related because they each measure aspects of your red blood cells.

    If the measures in these three areas are lower than normal, you have anemia. Anemia causes fatigue and weakness. Anemia has many causes, including low levels of certain vitamins or iron, blood loss, or an underlying condition.

    A red blood cell count that's higher than normal (erythrocytosis), or high hemoglobin or hematocrit levels, could point to an underlying medical condition, such as polycythemia vera or heart disease.

  • White blood cell count: A low white blood cell count (leukopenia) may be caused by a medical condition, such as an autoimmune disorder that destroys white blood cells, bone marrow problems or cancer. Certain medications also can cause white blood cell counts to drop.

    If your white blood cell count is higher than normal, you may have an infection or inflammation. Or, it could indicate that you have an immune system disorder or a bone marrow disease. A high white blood cell count can also be a reaction to medication.

  • Platelet count: A platelet count that's lower than normal (thrombocytopenia) or higher than normal (thrombocytosis) is often a sign of an underlying medical condition, or it may be a side effect from the medication.

    If your platelet count is outside the normal range, you'll likely need additional tests to diagnose the cause.

For specifics about what your complete blood count results mean if they fall outside the normal ranges, talk to your doctor.