This is a blood test in which the percentage of carbon monoxide bound hemoglobin is measured. Carbon monoxide's affinity to bind to hemoglobin is 200 times more than that of oxygen. Carboxyhemoglobin level is elevated in smoke inhalation (such as fire in an enclosed space), car exhaust inhalation, and smoking. Reference range: non smokers less than 3% and smokers greater than 12%