Blood differential
Alternative names:
differential; white blood cell differential count
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 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 experience, 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 and is used to differentiate the type of response in progress. 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-type lymphocytes (T cells), B-type 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 cytokines. B-type 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 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.
|
