Antithrombin III is a blood test that measures the amount of
antithrombin III (AT III), a protein that helps control blood
clotting. How the test is performed Blood is typically drawn
from a vein, usually from the inside of the elbow or the back of
the hand. The site is cleaned with germ-killing medicine
(antiseptic). The health care provider wraps an elastic band
around the upper arm to apply pressure to the area and make the
vein swell with blood. Next, the health care provider gently
inserts a needle into the vein. The blood collects into an
airtight vial or tube attached to the needle. The elastic band
is removed from your arm. Once the blood has been collected, the
needle is removed, and the puncture site is covered to stop any
bleeding. In infants or young children, a sharp tool called a
lancet may be used to puncture the skin and make it bleed. The
blood collects into a small glass tube called a pipette, or onto
a slide or test strip. A bandage may be placed over the area if
there is any bleeding. How