A person's blood count is tested by undergoing a complete blood count, explains Mayo Clinic. During a complete blood count, a health care professional takes a blood sample by placing a needle into a person's vein, usually at the arm. The sample is then sent to a laboratory for analysis. This test is used to measure several blood components, evaluate a person's general condition, and to identify several disorders, such as leukemia, infections and anemia.
The CBC test is performed via venipuncture, and blood is drawn from a vein found at the back of the hand or the inside of the elbow, according to MedlinePlus. A health care professional first cleans the site with an antiseptic before wrapping an elastic band around the upper arm. This band causes the vein to swell with blood. Once the needle is inserted into the vein, the blood is drawn into a tube or vial. The health care professional releases the elastic band, withdraws the needle, and covers the area with a bandage to stop the bleeding.
A lancet is often used in younger children or infants to puncture the skin and cause bleeding, explains MedlinePlus. The blood is then placed onto a test strip or a slide. If there is any visible bleeding, a health care professional places a bandage over the affected area.