Perhaps the most common blood test, a CBC provides pieces of information about a person's state of health based on the number of red blood cells, white blood cells and platelets present in the blood.
This blood test can be performed in a clinical laboratory, hospital or physician's office and requires no special preparation on the part of the patient.
A tourniquet or rubber strap is tied around the upper arm to mildly restrict the flow of blood and keep blood in the vein. Then, a needle with an attached tube-like container is inserted into a vein, usually in the bend of the elbow or the top of the hand. After a sufficient sample of blood is obtained, the needle is withdrawn, a bandage is placed on the puncture site and firm pressure is held until the bleeding stops.
In newborn infants and small children, blood may be withdrawn by pricking a finger since their veins may be difficult to find.
The entire test takes less than five minutes and produces only mild discomfort. The patient may experience slight bruising at the puncture site following this test but they can resume.