The health care provider wraps a tourniquet or blood pressure cuff around your upper arm. This creates pressure that causes veins below the area to fill with blood. After cleaning the area with a germ killer (antiseptic), the person performing the test will place a need into your vein, usually near the inside of the elbow or back of the hand. A thin tube, called a catheter, is then placed into the vein. (It may be called an IV, which means intravenous.) While the tube stays in place, the needle and tourniquet are removed. A dark green powder called methylene blue goes through the tube into your vein. The health care provider looks at how the powder turns a substance in the blood called methemoglobin into normal hemoglobin. | 
Tests
