Cut-off levels for breath carbon monoxide as a marker for cigarette smoking

Aims Current clinical studies often use a breath carbon monoxide (BCO) cutoff level of 8 parts per million (p.p.m.) or higher to identify smoking. In this study, the cut-off level of BCO as an indicator of smoking over the past 24 hours was re-examined. Design BCO and self-reported smoking were obtained each weekday for up to 14 weeks in 213 subjects paid to deliver reduced BCO values. Analysis of 12 3 8 6 paired values for reported smoking and BCO were analyzed. Findings The 25% quartile, median and 75% quartile values for BCO were 1, 1 and 2 p.p.m. on non-smoking days and 2, 5 and 12 p.p.m. on smoking days. respectively. Receiver-operating characteristic (ROC) analysis indicated that BCO provided high diagnostic accuracy to distinguish between smoking and non-smoking days [area under the curve (AUC)=0.853, P<0.0001]. The highest combined sensitivity and specificity was observed at a BCO cut-off level of 3 p.p.m. (sensitivity = 71.5%; specificity = 84.8%). At a BCO cut-off of 8 p.p.m. sensitivity and specificity were 40.6% and 98.2%, respectively, indicating that many smokers would be falsely classified as abstinent. Finally, the percentage of true tests (positive and negative) was highest at a BCO cut-off of 2 p.p.m. (80.20%). Conclusions BCO cut-off levels well below 8 p.p.m and as low as 2-3 p.p.m. may be more useful when it is important to maximize identification of smoking abstinence with a high degree of certainty.