White blood cells are an important part of your body’s immune system. They’re responsible for protecting your body against infections and invading organisms. You have five types of white blood cells:

  • neutrophils
  • lymphocytes
  • monocytes
  • eosinophils
  • basophils

Each of these can be affected in different ways if you have a particular condition or disease.

A white blood cell (WBC) count measures the number of white blood cells in your blood, and 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 and abnormalities, both of which are signs of potential issues.

A WBC count can also be called a leukocyte count, and a WBC differential can also be called a leukocyte differential count.

Read more: WBC (white blood cell) count »

Your doctor may request a WBC count and differential if they suspect you have one of several conditions, including:

  • anemia
  • infection
  • leukemia

Both tests can help your doctor determine if your symptoms are due to high or low WBC levels, which will help them understand what condition you may have. WBC counts can also be used to monitor certain disease processes and illnesses.

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.

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 in your hand. Once the blood is collected, it’s sent to a lab for analysis.

WBC count and differential tests have very few risks. Apart from bruising or soreness at the puncture site, this test shouldn’t cause any problems or complications. Some people may feel moderate pain and a stinging sensation during the blood draw, and some people may feel sick or lightheaded during or after the blood draw. If you do, let your doctor or nurse know and remain seated until the feeling has passed.

Although rare, some people who have blood drawn can develop a hematoma — an accumulation of blood directly under the skin. Some people may also develop an infection in the skin at the collection site, but this is also extremely rare.

Depending on which tests your doctor ordered, you may have to wait several days for results. A single WBC count or differential test doesn’t tell the whole story of what’s going on in your body. However, both tests are important tools that help your doctor find out what might be causing your symptoms. Differential results may indicate certain conditions, which are discussed below.

An increase in neutrophils in your blood may be caused by:

A decrease in neutrophils in your blood may be caused by:

An increase in lymphocytes in your blood may be caused by:

A decrease in lymphocytes may be caused by:

  • chemotherapy
  • HIV infection
  • leukemia
  • sepsis
  • radiation exposure, either accidental or from radiation therapy

An increase in monocytes may be caused by:

  • chronic inflammatory disease
  • tuberculosis
  • viral infection, such as measles, mononucleosis, and mumps

A decrease in monocytes may be caused by:

  • bloodstream infection
  • chemotherapy
  • bone marrow disorder
  • skin infections

An increase in eosinophils may be caused by:

A decrease in basophils may be caused by acute allergic reaction.

Your doctor will go over the test results with you and, if need be, come up with a treatment plan suited specifically for you. You may need to have more tests performed to confirm a diagnosis, and you may need to get another WBC count and differential in the near future.