This test measures the number of white blood cells, or WBCs, in your blood. Cells in your bone marrow produce white blood cells and release them into the bloodstream to help you fight infection. White blood cells are part of the 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 and threaten to make you sick.
When you get sick, your white blood cell count is higher than normal because your body is releasing more of these cells to fight the infection. But if you have certain illnesses like HIV or cancer, or if are on medication like chemotherapy that weakens your immune system, your white blood cell count can drop to very low levels.
White blood cells are divided into five major types:
This test measures the total count of all types of white blood cells. It does not measure the levels of individual types of white blood cells.
You may need this test to determine if you have an infection or illness that is affecting your immune system. If your immune system is weakened by medication or illness, you may also need this test to see if your white blood cell count is dangerously low. In that case, even a simple infection could be quite harmful because your body isn't able to defend itself.
In addition to a white blood cell count, your doctor may order a differential white blood cell count. This blood test measures the amount of each type of white blood cells. You may also need a complete blood count, or CBC, which measures all of the major blood cells, including white blood cells. A neutrophil test 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.
Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.
Normal white blood cell counts are:
9,000 to 30,000/mm3 for newborns
6,200 to 17,000/mm3 for children under 2 years old
5,000 to 10,000/mm3 for children older than age 2 up to 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 and that even a minor infection could cause serious health problems.
The test requires a blood sample, which is drawn through a needle from a vein in your arm. Infants usually have one of their heels stuck with a needle to collect a few drops of blood.
Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.
Some medications may affect your test results. Be sure your doctor knows about treatments you are receiving, medications you are taking, or recent illnesses you've had.
A blood test rarely requires any preparation. You can probably eat, drink, and take your medication as usual, but check with your doctor to be sure. Be sure your doctor knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use.