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The differential white blood cell count measures how many different kinds of white blood cells are in the bloodstream. As part of this test, the technologist looks to see if the structure of the white blood cells is normal or abnormal.

Who is a candidate for the test?

This test is ordered to check for infection, anemia or leukemia. It can also be used to monitor the course of treatment in these conditions.

How is the test performed?

A blood sample is taken from a vein on the forearm or hand. First, the skin over the vein is cleaned with an antiseptic. Next, a strong rubber tube, or "tourniquet," is wrapped around the upper arm. This enlarges the veins in the lower arm by restricting blood flow through them.

A fine needle is gently inserted into a vein, and the tourniquet is removed. Blood flows from the vein through the needle, and is collected in a syringe or vial. After the needle is withdrawn, the puncture site is covered with a bandage for a short time to prevent bleeding.

In the laboratory, a drop of the blood is placed on a microscope slide. A blood smear is made and stained. A laboratory technologist observes the slide under a microscope and counts the numbers of each kind of cell per high power field.

Automated blood cell counting machines, the kind used for hemoglobin values and red cell counts, can estimate numbers of some types of white blood cells, but will miss certain other types and cannot give a description of what the abnormal cells look like.

What is involved in preparation for the test?

Specific instructions are available from a healthcare professional.

What do the test results mean?

At least five different types of white blood cells are normally found in a person's blood -- neutrophils, lymphocytes, monocytes, eosinophils, and basophils - listed in order from most numerous in a normal blood sample, to least numerous. Each of them plays a different role. Some destroy foreign cells and organisms. Some make antibodies that attack cells that are infected with a virus. Some destroy unwanted cells and organisms.

Certain findings on the differential test are likely to mean:

• A higher percentage of neutrophils may indicate acute infection, eclampsia, gout, myeloid leukemia, rheumatoid arthritis, rheumatic fever, acute stress, thyroiditis, trauma, or a reaction to certain medications.
• A lower percentage of neutrophils may indicate aplastic anemia, some types of bacterial infection, chemotherapy, influenza or radiation therapy.
• An increased percentage of lymphocytes may indicate chronic bacterial infection, infectious hepatitis, infectious mononucleosis, lymphocytic leukemia, multiple myeloma, viral infection, or recovery from a bacterial infection.
• A percentage of lymphocytes lower than normal may indicate chemotherapy, HIV infection, leukemia, radiation therapy or sepsis.
• A higher percentage of monocytes may indicate chronic inflammatory disease, parasitic infection, tuberculosis or viral infection.
• An increased percentage of eosinophils may indicate allergic reaction, parasitic infection or Hodgkin's disease.
• A decreased percentage of basophils may mean an acute allergic reaction.