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


Martin Javors, Department of Psychiatry, The University of Texas HSC, 7703 Floyd Curl Drive, San Antonio, Texas 78229, USA, Tel: 210 567 5532, Fax: 210 567 3759, E-mail:


Aims  Current clinical studies often use a breath carbon monoxide (BCO) cut-off 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 12 386 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. (80.2%).

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.