This is usually drawn from a vein in the forearm or hand. To do this, the skin over the vein is first cleaned with an antiseptic. Next, a strong rubber tube, or tourniquet, is wrapped around the upper arm. This enlarges the veins in the lower arm by restricting blood flow through them. A very thin needle is gently inserted into a vein and the tourniquet is removed.
Blood flows from the vein through the needle and is collected in a syringe or vial.
It is then sent to the laboratory for glucose testing. After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding. In some cases, urine samples may also be used to look for extra glucose. Samples of midstream urine, that is, not at the beginning and not at the end, are collected at the same time intervals as the blood samples are taken. People are often advised not to eat for at least 8 hours before the test.