Blood Test: Complete Blood Count





Blood Test: Complete Blood Count
What It Is



The complete blood count (CBC) is a common blood test that evaluates
the three major types of cells in the blood: red blood cells, white
blood cells, and platelets.

Why It's Done



A CBC may be ordered as part of a routine checkup, or if your child
is feeling more tired than usual, seems to have an infection, or has
unexplained bruising or bleeding.

• Red blood cells: The CBC's measurements of red
  blood cell (RBC) count, hemoglobin (the oxygen-carrying protein in
  RBCs), and mean (red) cell volume (MCV) provides information about the
  RBCs, which carry oxygen from the lungs to the rest of the body. These
  measurements are usually done to test for anemia, a common condition
  that occurs when the body has insufficient red blood cells.
  
• White blood cells: The white blood cell (WBC) count
  measures the number of WBCs (also called leukocytes) in the blood. The
  WBC differential test measures the relative numbers of the different
  kinds of WBCs in the blood. WBCs, which help the body fight infection,
  are bigger than red blood cells and there are far fewer of them in the
  bloodstream. An abnormal WBC count may indicate an infection,
  inflammation, or other stress in the body. For example, a bacterial
  infection can cause the WBC count to increase, or decrease,
  dramatically.
  
• Platelets: The smallest blood cells, platelets play
  an important role in blood clotting and the prevention of bleeding.
  When a blood vessel is damaged or cut, platelets clump together and plug
  the hole until the blood clots. If the platelet count is too low, a
  person can be in danger of bleeding in any part of the body.
  

The CBC can also test for loss of blood, abnormalities in the
production or destruction of blood cells, acute and chronic infections,
allergies, and problems with blood clotting.

Preparation



No special preparations are needed. Having your child wear a
short-sleeve shirt on the day of the test can make things easier for the
technician who will be drawing blood.The Procedure



Not much blood is drawn in a CBC. A health professional will usually
draw the blood from a vein. For an infant, the blood may be obtained by
puncturing the heel with a small needle (lancet). If the blood is being
drawn from a vein, the skin surface is cleaned with antiseptic, and an
elastic band (tourniquet) is placed around the upper arm to apply
pressure and cause the veins to swell with blood. A needle is inserted
into a vein (usually in the arm inside of the elbow or on the back of
the hand) and blood is withdrawn and collected in a vial or syringe.

After the procedure, the elastic band is removed. Once the blood has
been collected, the needle is removed and the area is covered with
cotton or a bandage to stop the bleeding. Collecting blood for this test
will only take a few minutes.



What to Expect



Either method (heel sticking or vein withdrawal) of collecting a
sample of blood is only temporarily uncomfortable and can feel like a
quick pinprick. Afterward, there may be some mild bruising, which should
go away in a few days.

Getting the Results



The blood sample will be processed by a machine. Parts of the CBC
results can be available in minutes in an emergency, but more commonly
the full test results come after a few hours or the next day.

If a CBC test points to anemia, infection, or other concerns, your
child's doctor may repeat the test just to be sure. If the second set of
test results come back the same, your doctor will likely order further
lab tests for your child to determine what's causing the problem and how
to treat it.

Risks



The CBC test is considered a safe procedure. However, as with many
medical tests, there are some problems that can occur with having blood
drawn, such as:

• fainting or feeling lightheaded
  
• hematoma (blood accumulating under the skin causing a lump or a bruise)
  
• pain associated with multiple punctures to locate a vein
  

Helping Your Child



Having a blood test is relatively painless. Still, many kids are
afraid of needles. Explaining the test in terms your child can
understand might help ease some of the fear.

Allow your child to ask the technician any questions he or she might
have. Tell your child to try to relax and stay still during the
procedure, as tensing muscles and moving can make it harder and more
painful to draw blood. It also may help if your child looks away when
the needle is being inserted into the skin.

If You Have Questions



If you have questions about the CBC test, contact your doctor.