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Ency. home > Test > B > Blood differential

Blood differential    See images

Overview | Risks | Results

Alternative names:

Differential; White blood cell differential count

Definition:

A test that 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

Adult or child:
Blood is 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 air-tight 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.

Infant or young child:
The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

A drop of blood is placed on a glass slide and a smear is made and stained to differentiate the various types of WBCs for a manual differential (the cells are counted by a technician, hematologist, or pathologist rather than a computer).

How to Prepare for the Test

Adult:
No special preparation is necessary.

Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:

How the Test will Feel

When 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 estimates the number of the various types of WBCs. This can help detect infection, anemia, and leukemia; or follow the progress of treatment.

There are various types of WBCs (also called leukocytes) that normally appear in the blood: neutrophils (polymorphonuclear leukocytes; PMNs), band cells (slightly immature neutrophils), T lymphocytes (T cells), B lymphocytes (B cells), monocytes, eosinophils, and basophils. The differential quantifies the relative percentages of the different types of cells in the blood and also notes any abnormal appearance (morphology) of the cells or the presence of any abnormal immature cells.

Neutrophils are mainly phagocytic cells (that is, they engulf and destroy invading organisms); they also release some enzymes and substances that affect the function of other cells called cytokines. B lymphocytes synthesize and secrete antibodies. One type of T cell is called a helper cell; it secretes proteins that increase B cell function and the functions of cytotoxic T lymphocytes. Cytotoxic T lymphocytes recognize and destroy virus-infected and other abnormal cells. Monocytes recognize a variety of microorganisms, especially gram-negative bacteria. Activated monocytes turn into macrophages that can engulf and destroy microorganisms and secrete a variety of cytokines that modulate the activity of other leukocytes. Eosinophils and basophils release substances that cause vasoconstriction, smooth muscle contraction, and an increased permeability of small blood vessels. Eosinophils are stimulated by parasites and some bacteria. Basophils are stimulated by allergens.

Ency. home > Test > B > Blood differential

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