Possible reasons for elevated carbon monoxide levels in self-reported ex-smokers.

Expired-air carbon monoxide (CO) is often used to validate self-reported nonsmoking in clinical settings and research studies. It sometimes occurs that a person reports to have quit smoking, whereas the CO measurement classifies him/her as a smoker. This commentary summarizes possible explanations for this discrepancy and briefly discusses what to do when there is reason to believe that the CO measurement is incorrect.