It is important to note that many factors can influence the results of blood tests. Often, certain drugs, foods, exercise, menstruation, smoking, etc. can raise or lower levels of certain substances, and alter results. Results also often vary due to race, gender and age. Different labs use different methods, and may have different results with different meanings. Furthermore, most labs set the normal result range so that 95 percent of healthy patients will have a normal test result. This means that on average, one in twenty healthy people will show an abnormal test result, or false positive. It is important to find out from your doctor what factors can influence the results of your test.
Blood is usually drawn from a vein in the arm, near the inside of the elbow. The physician ties a tourniquet tightly around the upper arm, so that veins below it fill with blood. After the area is cleaned with antiseptic, a needle is inserted at about a 45 degree angle from the lower arm. The blood is collected in an airtight syringe to prevent contamination. Before removing the needle, the doctor removes the tourniquet from the arm. This prevents blood from immediately being pushed out of the body due to the pressure from the tourniquet. When enough blood is collected in the syringe, the physician removes the needle and applies a small bandage or cotton to absorb any bleeding.
Last updated: 28-Jul-08