The report by Tappin et al on the financial incentives for smoking shows an impressive reduction in smoking at the end of pregnancy1. This was a well executed study, with important implications for further initiatives to reduce the damage caused by smoking.
My concern is the over reliance on the use of expired-air carbon monoxide (eCO) as a means to verify self-reported abstinence from tobacco; using a cut off of 7 parts per million for the basis on which to allocate shopping vouchers to the value of more than £200.
The problem is that eCO is increasingly questioned as a reliable means of assessing abstinence from tobacco2. The half-life of CO is about 3 hours, meaning eCO only measures smoking habit for a 6-8 hour period, thus many smokers can abstain perhaps for just a few hours and the test will be negative. This was one point highlighted by the current study, stating ‘a small number of self reported quitters were validated as non-smokers by carbon monoxide breath testing but may have only temporarily abstained’. One study estimated that about 40% of those who smoked within 24 hours had eCO reading below 10 ppm and concluded that an eCO criterion of 5 ppm may be optimal to validate 24-hr cessation and reduce misclassification of smokers as “abstinent.”3.
While the use of eCO is useful as part of the Stop Smoking Services for participants to compare results and to provide feedback about their efforts to quit, more accurate and scientific methods should be employed to biochemically verify abstinence for research projects, and for issuing financial incentives. For example urinary and salivary cotinine testing, which was used in the current study only to verify cessation at 34-38 weeks can be carried out with inexpensive point of care methods4.
This study claimed that ‘A possible unintended consequence of financial incentives was that women were untruthful about their smoking status when asked during the trial, especially at the time of the primary outcome assessment’, yet financial incentives were allotted on the basis of a test which can be deceived by refraining from smoking overnight or less.
1. Tappin D, Bauld L, Purves D et al. Financial incentives for smoking cessation in pregnancy: randomised controlled trial. BMJ 2015; 350: h134 doi: 10.1136/bmj.h134
2. Schober P, Schwarte LA, Loer SA. Exhaled carbon monoxide concentration: a reliable predictor of smoking status? Eur J Anaesthesiol 2011; 28: 146-147
3. Perkins KA, Karelitz JL, Jao NC. Optimal carbon monoxide criteria to confirm 24-hour smoking abstinence. Nic Tob Res 2013; 15: 978-982.
4. Cope GF, Wu HHT, O’Donovan GV, Milburn HJ. A new point of care cotinine test for saliva to identify and monitor smoking habit. Eur Respir J 2012; 40: 496-7
Competing interests: GF Cope is the inventor of a point of care urinary and salivary cotinine test called SmokeScreen and he is a director of the manufacturer, GFC Diagnostics Ltd.