The Carbon Monoxide, Blood (Carboxyhemoglobin) is used to determine the extent of carbon monoxide poisoning, toxicity; check on the effect of smoking on the patient; work up headache, irritability, nausea, vomiting, vertigo, dyspnea, collapse, coma, convulsions; work up persons exposed to fires and smoke inhalation. Carboxyhemoglobin is useful in judging the extent of carbon monoxide toxicity and in considering the effect of smoking on the patient. A direct correlation has been claimed between CO level and symptoms of atherosclerotic diseases, intermittent claudication, angina, and myocardial infarction. Exposure may occur not only from smoking but also from garage exposure, and from various motors. This test may be included when blood gases are ordered, when there is sufficient sample, and when such instrumentation is available. A danger of missed diagnosis of CO intoxication is continued exposure of the patient and others to a toxic environment. The cherry red color of CO poisoning is not consistently seen. CO intoxication may contribute to the risk of myocardial infarction. A strong correlation is present between carboxyhemoglobin levels and psychometric testing abnormalities.Psychometric testing measures actual neurologic disability and may therefore better define carboxyhemoglobin poisoning severity than blood CO level.