CARBOXYHEMOGLOBIN LEVEL - A blood test which is performed on an arterial specimen and is a measurement of the amount of carbon monoxide which is present and bound to hemoglobin. This is an important test to diagnosis carbon monoxide toxicity (smoke inhalation). Normal carboxyhemoglobin may be from 3% to as high as 12% in heavy smokers. Symptoms of carbon monoxide poisoning begin at about 20% carboxyhemoglobin.