Does the Finding of Small Airway Obstruction in Pulmonary Function Tests Effect End-Expiratory Carbonmonoxide Level?

Objective: End-expiratory CO level, which is used to test smoking cessation is known to be effected by many factors. We aimed to evaluate effect of small airway obstruction (SAO) on end-expiratory CO levels.
Methods: The study population consisted of cases who applied to smoking cessation clinic between April-December 2014. The duration of smoking (pack-years), pulmonary function tests (PFT) and end-expiratory carbonmonoxide (CO) were noted at first visit. The cases were questioned for last time of smoking and amount of cigarettes they smoked within past 12 hours. Pearson correlation test was used to determine factors effecting CO levels.
Results: Mean age of 167 cases -112 male (67.1%), 55 female (32.9%) was 38.7±12.7 years. Mean duration of smoking was 23.3±16.4 pack-years, mean CO level was 14.1±6.9 ppm, mean FEF25-75% was 77.2±28.2. Mean period elapsed after last cigarette was 1.5±1.8 hours, mean number of cigarettes smoked within past 12 hours was 7.2±5.0. SAO was found in 50 cases (34.1%). Correlation analyses revealed that CO levels were correlated positively to duration of smoking and amount of cigarettes they smoked within past 12 hours (p=0.03, r=0.228; p=0.000, r=0.511, respectively) and negatively to the last time of smoking (p=0.023, r=0.176). SAO and in PFT was not correlated to CO level (p>0.05).
Conclusion: End-expiratory CO level is closely related to duration of smoking, number of cigarettes smoked within past 12 hours and period elapsed after last cigarette. Presence of SAO does not seem to effect CO levels.