White Blood Count (WBC)

Why this test may be ordered

• To help diagnose or monitor disorders related to a high or low white blood cell count
• If you have signs of infection, inflammation, or autoimmune disease
• To monitor treatment if you have a disease that weakens your immune system or take medicine that lowers your immune response
• As part of a routine screening for newborns or children with symptoms of a white blood cell disorder

Preparation and collection

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.

A health care professional will take a blood sample from a vein in your arm, using a small needle. For newborns and young babies, a sample may be taken from the heel; for older babies and children, from the fingertip or arm. The site is cleaned with alcohol and poked with a small needle, and a bandage is applied.

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 and a small bruise may form, which should go away quickly.

Understanding results

Your white blood count result will show whether the number of white blood cells in your blood is high, low, or within the expected range. However, this test alone cannot confirm a diagnosis. Your provider will interpret your result along with your symptoms, medical history, and other tests.

• High (leukocytosis): Possible infection, inflammatory disease (such as rheumatoid arthritis), allergy, leukemia, Hodgkin disease, tissue damage from a burn or surgery, smoking, stress, reaction to a medicine, or pregnancy. Limitation: A high white blood count alone cannot confirm a specific condition; other tests are needed to determine the cause. Next step: Your provider will consider your symptoms, medical history, and results of other blood tests to understand the cause of the high count.
• Low (leukopenia): Possible bone marrow damage (from infection, disease, or treatments such as chemotherapy), cancers that affect the bone marrow, autoimmune disorder (such as lupus), or HIV. Limitation: A low white blood count alone cannot confirm a specific condition; other tests are needed to determine the cause. Next step: Your provider will consider your symptoms, medical history, and results of other blood tests to understand the cause of the low count.

• A white blood count can show if the number of your white blood cells is too high or too low, but it cannot confirm a diagnosis.
• Results are usually interpreted along with other tests, such as a complete blood count, blood differential, blood smear, or bone marrow test.
• Certain medicines, smoking, stress, and pregnancy can affect your white blood count.
• Your provider will consider your symptoms and medical history when interpreting results.

What to do next

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 and determine if any follow-up is needed.

• What is a white blood count and what does it measure?
• Why might I need this test?
• How is the blood sample collected?
• Do I need to do anything to prepare for the test?
• What do the results mean and what are the next steps?

Evidence source

U.S. National Library of Medicine: White Blood Count (WBC)