A white blood cell (WBC) count measures the number of white blood cells in your blood. A WBC differential determines the percentage of each type of white blood cell present in your blood. A differential can also detect immature white blood cells or any abnormalities, both of which are signs of a potential problem.
A WBC count may also be called a leukocyte count, and a WBC differential count may be called a leukocyte differential count.
White blood cells are an important part of your body’s immune system: They are responsible for protecting your body against infections and invading organisms. You have five types of white blood cells: neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Each of these is affected in a different way depending on the condition or disease that is affecting your WBC counts.
Your doctor may request a WBC count and differentials if he or she suspects you have one of several conditions, including:
Both the WBC count and WBC differential can provide your doctor with clues as to why you have elevated or low white blood cell numbers. Both tests can also help your doctor know if your condition is getting worse or improving. WBC counts that return to normal indicate a condition is improving. WBC counts that do not change or worsen indicate a condition that is not being treated properly.
To conduct a WBC count and differential, your doctor will need to collect a blood sample from you. Blood is typically drawn from a vein in either the bend of your arm or on your hand. (In children and infants, blood can be taken from a heel or finger stick.) Your healthcare provider may use a tourniquet to restrict blood flow and help the veins in your arm provide a sufficient blood sample.
Once the blood is collected, it is sent to a lab for analysis.
WBC count and differential tests have very few potential risks. Apart from bruising or soreness at the puncture site, this test should not cause any problems or complications. Some people may feel moderate pain and a stinging sensation during the blood draw. Some people may feel sick or lightheaded during or after the blood draw. If you do, let your healthcare provider know, and remain seated until the feeling has passed.
Rarely, people who have had blood drawn develop a hematoma, an accumulation of blood directly under the skin. People who have blood collected may also develop an infection as a result of the collection, but this is also rare.
No special preparation is necessary for a WBC count or differential. Your doctor may ask you to stop taking certain medications, including over-the-counter supplements or vitamins, for several days before the sample collection occurs. Medicines, both prescription and over-the-counter, can affect white blood cell counts.
Once your healthcare provider has enough blood to conduct a WBC count and differential, he or she will remove the needle and place moderate pressure on the injection site to stop any bleeding. The blood is shipped directly to a lab for analysis. Depending on which tests your doctor ordered, you may wait several days for results.
A single WBC count or differential test does not tell the whole story of what’s going on with your body. However, both tests are important tools for doctors looking to find out what might be causing your symptoms. These differential results may indicate certain conditions, as is discussed below.
An increase in neutrophils in your blood may be caused by:
- acute stress
- an infection
- rheumatoid arthritis
A decrease in neutrophils in your blood may be caused by:
- bacterial infection
- radiation exposure
An increase in lymphocytes in your blood may be caused by:
- chronic infection
- an infectious viral infection, such as the mumps or measles
A decrease in lymphocytes may be caused by:
- HIV infection
- radiation exposure (from radiation therapy or accidental)
An increase in monocytes may be caused by:
- chronic inflammatory disease
- viral infection, such as measles, mononucelosis, and mumps
An increase in eosinophils may be caused by:
- an allergic reaction
- parasitic infection
A decrease in basophils may be caused by acute allergic reaction.