What is a white blood count (WBC)?

A white blood count measures the number of white blood cells (WBCs) in your blood. White blood cells, also called leukocytes, are part of your immune system. They are a type of blood cell made in your bone marrow and found in your blood and lymph tissue (part of your immune system). If you are injured or get sick, the white blood cells will travel through your bloodstream and tissues to where they are needed. There, they will help your body fight off infections and other diseases.

When you get sick, your body makes more white blood cells to fight the bacteria, viruses, or other foreign substances causing your illness. This increases your white blood count.

Other diseases can cause your body to make fewer white blood cells than you need. This lowers your white blood count. Diseases that can lower your white blood count include some types of cancer and HIV, a viral disease that attacks white blood cells. Certain medicines, including chemotherapy, may also lower the number of your white blood cells.

There are five major types of white blood cells:

• Neutrophils
• Lymphocytes
• Monocytes
• Eosinophils
• Basophils

A white blood count measures the total number of these cells in your blood. Another test, called a blood differential, measures the amount of each type of white blood cell.

Other names: WBC count, white cell count, white blood cell count, Leukocyte Count, WBC

What is it used for?

A white blood count is most often used to help diagnose or monitor disorders related to having a high white blood cell count or low white blood cell count.

Disorders related to having a high white blood count include:

• Autoimmune and inflammatory diseases, conditions that cause the immune system to attack healthy tissues
• Bacterial or viral infections
• Cancers such as leukemia and Hodgkin disease
• Allergic reactions

Disorders related to having a low white blood count include:

• Diseases of the immune system, such as HIV
• Lymphoma, a cancer of the bone marrow
• Diseases of the liver or spleen

A white blood count can show if the number of your white blood cells is too high or too low, but it can't confirm a diagnosis. So it is usually done along with other tests to help confirm your diagnosis. These other tests could include a complete blood count, blood differential, blood smear, and/or bone marrow test.

Why do I need a white blood count?

You may need this test if you have signs of an infection, inflammation, or autoimmune disease. The symptoms of an infection may include:

• Fever
• Chills
• Body aches
• Headache
• Wound that is red, has pus, or won't heal
• Ongoing cough

The symptoms of inflammation and autoimmune diseases will be different, depending on where in the body the inflammation is and the type of disease you have.

You may also need this test if you have a disease that weakens your immune system or are taking medicine that lowers your immune response. If the test shows your white blood count is getting too low, your provider may want to adjust your treatment.

Your newborn or older child may also be tested as part of a routine screening or if they have symptoms of a white blood cell disorder.

What happens during a white 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.

To test children, a health care provider will take a sample from the heel (newborns and young babies) or the fingertip or arm (older babies and children). The provider will clean the heel, fingertip or arm with alcohol and poke the site with a small needle. The provider will collect a blood sample and put a bandage on the site.

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

You don't need any special preparations for a white blood count. You may need to stop taking certain medicines before this test, so tell your provider about everything you take. But don't stop taking any medicines unless your provider tells you to.

Are there any risks to the test?

After a blood test, you may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.

There is very little risk to your baby or child with a needle stick test. Your child may feel a little pinch when the site is poked, and a small bruise may form at the site. This should go away quickly.

What do the results mean?

Conditions that may cause a high white blood count, also called leukocytosis, include:

• An infection
• An inflammatory disease such as rheumatoid arthritis
• An allergy
• Leukemia or Hodgkin disease
• Tissue damage from a burn injury or surgery

Smoking, stress, a reaction to a medicine, or pregnancy may also cause your body to make more white blood cells, which leads to a high white blood count.

Conditions that may cause a low white blood count, also called leukopenia, include:

• Bone marrow damage. This may be caused by infection, disease, or treatments such as chemotherapy.
• Cancers that affect the bone marrow.
• An autoimmune disorder, such as lupus.
• HIV.

If you are already being treated for a white blood cell disorder, your results may show if your treatment is working or whether your condition has improved.

If you have questions about your results, talk to your provider. Your provider will consider your symptoms, medical history, and the results of other blood tests to understand your white blood count results.

Learn more about laboratory tests, reference ranges, and understanding results.

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

White blood count results are often compared with results of other blood tests, including a blood differential. A blood differential test shows the amount of each type of white blood cell, such as neutrophils or lymphocytes. Neutrophils mostly target bacterial infections. Lymphocytes mostly target viral infections.

• A higher-than-normal amount of neutrophils is known as neutrophilia.
• A lower-than-normal amount is known as neutropenia.
• A higher-than-normal amount of lymphocytes is known as lymphocytosis.
• A lower-than-normal amount is known as lymphopenia.