 - Self-report and biochemical verification are used to determine smoking status in treatment trials and clinical research. Each method has merits and limitations that make it appropriate for particular situations. Participants who feel social pressure to report tobacco abstinence may provide unreliable self-reporting results. Biochemical verification using breath carbon monoxide CO is a more reliable indicator, but several biological and environmental factors including exposure to secondhand smoke can affect the sensitivity and specificity of breath CO measurement. 