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Alternative namesDifferential; White blood cell differential count
DefinitionThe blood differential test measures the relative numbers of white blood cells (WBCs) in the blood. It also includes information about abnormal cell structure and the presence of immature cells. (See also CBC , peripheral smear, and eosinophil count - absolute .)
How the test is performed
Blood will be drawn from a vein ( venipuncture ), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill with blood).
A needle is inserted into the vein, and the blood is collected in an airtight vial or a syringe. During the procedure, the tourniquet is removed to restore circulation.
Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding .
For infants or young children:
How to prepare for the test
No special preparation is necessary for adults.
For infants and young children:
How the test will feelWhen the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
Why the test is performed
The differential count can be used to help detect infection, anemia , and leukemia or to follow the progress of treatment.
There are various types of WBCs (also called leukocytes) that normally appear in the blood. The differential determines the relative percentages of the different types of cells in the blood, notes any abnormal appearance of the cells, and the presence of any abnormal immature cells.
What abnormal results mean
Any infection or acute stress results in increased production of WBCs. This usually entails an increased numbers of cells and an increase in the percent of immature cells (mainly band cells) in the blood. High WBC counts may indicate the presence of an inflammatory and immune response, or it may result from other conditions such as leukemia.
It is important to realize that an abnormal increase in one type of leukocyte can produce an apparent decrease in the percentage of other types.
An increased percentage of neutrophils may indicate:
A decreased percentage of basophils may indicate an acute allergic reaction.
This test may be performed under many other conditions as well.
What the risks are
Special considerationsVeins and arteries vary in size from one patient to another, and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
Update Date: 5/8/2003Marcia S. Brose, M.D., Ph.D., Division of Hematology/Oncology, Hospital of the University of Pennsylvania, Philadelphia, PA. Review provided by VeriMed Healthcare Network.
Last updated: Tue, 06 Jan 2009 00:20:03 GMT