Misleading qualification of active smokers based on carbon monoxide measurement of exhaled air

Type of funding sources: Public Institution(s). Main funding source(s): Centre of Postgraduate Medical Education

Carbon monoxide (CO) concentration of exhaled air is widely used in clinical trials in assessment of active smoking due to its feasibility and cost-effectiveness. The concentration of nicotine metabolite cotinine in urine is used as an objective index of smoking.

We aimed to compare self-reported smoking and smoking cessation (SC) with results of CO and cotinine concentration.

Patients after coronary angiography (n=62) were included in SC programme with a brief counselling. The self-reported smoking and SC was verified with urine cotinine assessment (NicAlert) and CO level in exhaled air (MicroCO). The criterium of SC was non-smoking (<1 cigarette ) in the last 7 days, NicAlert ≤2, MicroCO test≤6 parts per million (ppm).

The mean age of patients was 61 ± 7.7 years, 49 (79%) patients were men, with median load of smoking 10 pack-years. 40 patients were assessed with both methods at baseline and follow-up visit. At baseline all the patients declaring at least 10 cigarettes daily had Nic Alert 6, but MicroCO indicated non-smoking in 25 (62,5%) patients. At follow up visit after 1 month, 35 (56,5%) patients declared SC and there was numerically higher level of misleading results of CO concentration in the whole group (p=0.077) and significantly higher in the group of patients who self-reported continuation of smoking (p=0.009)(Table 1).

Despite widespread and cost-effective measurement of CO the result should be carefully interpreted. It accurately indicates non-smoking patients, but the measurement in active smokers might be misleading with a significant group of patients with false negative results. To accurately assess active smokers the CO measurement should be supported with other method such as measurement of cotinine levels.