Most of the time, blood is typically drawn from a vein located on the inside of the elbow or the back of the hand.
In infants or young children, a sharp tool called a lancet may be used to puncture the skin. The blood collects in a small glass tube called a pipette, or onto a slide or test strip. A bandage is put over the spot to stop any bleeding.
How to Prepare for the Test
Most of the time, you do not need to take special steps before this test. Tell your doctor the medicines you are taking, including ones you buy without a prescription. Some drugs may change the test results.
How the Test will Feel
You may feel slight pain or a sting when the needle is inserted. You may also feel some throbbing at the site after the blood is drawn.
Why the Test is Performed
You will have this test to find out how many WBCs you have. Your body produces more WBCs when you have an infection or allergic reaction. You can also have more WBCs when you are under stress.
The normal number of WBCs in the blood is 4,500-10,000 white blood cells per microliter (mcL).
Normal value ranges may vary slightly among different labs. Some laboratories use different measurements or may test different specimens. Talk to your doctor about your test results.
The examples above show the common measurements for results for these tests.
What Abnormal Results Mean
LOW WHITE BLOOD CELL (WBC) COUNT
A low number of WBCs is called leukopenia. A WBC count below 4500 is below normal
One type of white blood cell is the neutrophil. This type of white blood cell is important for fighting infections.
An adult with who has fewer than 1700 neutrophils in a microliter of blood has a low white blood cell count.
If there are fewer than 500 neutrophils in a microliter of blood, the risk for infection becomes even higher.
It may be due to:
Bone marrow deficiency or failure (for example, due to infection, tumor, or abnormal scarring)
Cancer treating drugs, or other medicines (see list below)
Certain autoimmune disorders such as lupus
Disease of the liver or spleen
Radiation treatment for cancer
Certain viral illnesses, such as Mono
Cancers that damage the bone marrow
Very severe bacterial infections
HIGH WHITE BLOOD CELL COUNT
A high number of WBCs is called leukocytosis. It may be due to:
Certain drugs or medications (see list below
Infections, most often those caused by bacteria
Inflammatory disease (such as rheumatoid arthritis or allergy)
Severe mental or physical stress
Tissue damage (for example, burns)
There may also be other less common reasons for this result. .
Drugs that may lower your WBC count include:
Anti thyroid drugs
Drugs that may increase WBC counts include:
Beta adrenergic agonists (for example albuterol)
Granulocyte colony stimulating factor
There is very little risk involved with having your blood taken.
People who have had their spleen removed (splenectomy) will always have a slightly higher number of WBCs.
Berliner N. Leukopenia and leukocytosis. In: Goldman L, Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 170.
David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by A.D.A.M. Health Solutions, Ebix, Inc., Editorial Team: David Zieve, MD, MHA, David R. Eltz, Stephanie Slon, and Nissi Wang.