White Blood Cell Count

Does this test have other names?

WBC count, leukocyte count

What is this test?

This test measures the number of white blood cells (WBCs) in your blood. White blood cells are also called leukocytes.

Your bone marrow makes white blood cells and release them into the bloodstream. White blood cells help you fight infection. They are part of your body's immune system, which keeps you healthy and makes you well when you get sick. White blood cells work to destroy any foreign virus, fungus, or bacteria that enter your body.

When you get sick, your white blood cell count is higher than normal. This is because your body is releasing more of these cells to fight the infection. But if you have certain illnesses like HIV or cancer, your white blood cell count can drop to very low levels. It can also drop if you are on medicine that weakens your immune system. This includes medicines, such as chemotherapy.

White blood cells are divided into 5 main types:

• Neutrophils

• Basophils

• Lymphocytes

• Monocytes

• Eosinophils

This test measures the total count of all types of white blood cells. It does not measure the levels of each type of white blood cells. 

Why do I need this test?

You may need this test to find out if you have an infection or illness. If your immune system is weakened by medicine or illness, you may also need this test to see if your white blood cell count is too low. If it is, even a simple infection could be very harmful to your body.

What other tests might I have along with this test?

You may also have these tests:

• Differential WBC count. This blood test measures the amount of each type of white blood cell.

• Complete blood count (CBC). This measures all of the major blood cells, including white blood cells.

• Neutrophil test. This may be done to check for neutropenia. If you have neutropenia, it means your neutrophil count is low and you can easily get an infection. 

• Bacterial and viral cultures. Your healthcare provider may also send samples of your blood, urine, sputum, and cerebral spinal fluid (CSF) to the lab to check for bacteria and viruses.

• Imaging tests. You may have imaging tests, such as an MRI or CT scan, to look for sources of infection.

• Biopsy. If your healthcare provider thinks you may have a type of blood cancer, you may need a biopsy to help figure out the cause of your abnormal WBC values.

What do my test results mean?

Test results may vary depending on your age, gender, health history, and other things. Your test results may be different depending on the lab used. They may not mean you have a problem. Ask your healthcare provider what your test results mean for you.

Normal white blood cell counts are:

• 9,000 to 30,000/mm3 for babies 0 to 2 weeks old

• 5,000 to 21,000/mm3 for babies 2 to 8 weeks old

• 5,000 to 19,000/mm3 for children 2 months to 6 years old

• 4,800 to 10,800/mm3 for children 6 to 18 years old

• 4,500 to 10,500/mm3 for adults

Test results that are higher than normal may mean that you have an infection or illness that your body is fighting. Test results that are lower than normal may mean that your immune system isn't working as well as it should. This means that even a small infection could cause serious health problems. 

How is this test done?

The test is done with a blood sample. A needle is used to draw blood from a vein in your arm or hand. Infants usually have one of their heels stuck with a needle to collect a few drops of blood. 

Does this test pose any risks?

Having a blood test with a needle carries some risks. These include bleeding, infection, bruising, and feeling lightheaded. When the needle pricks your arm or hand, you may feel a slight sting or pain. Afterward, the site may be sore.

What might affect my test results?

Some medicines may affect your test results. Tell your healthcare provider about treatments you are getting, medicines you are taking, or recent illnesses you've had. 

How do I get ready for this test?

You can probably eat, drink, and take your medicine as usual, but check with your healthcare provider. Be sure your provider knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illegal drugs you may use.