Normal values of exhaled carbon monoxide in healthy subjects: comparison between two methods of assessment

Background: In a previous study, exhaled carbon monoxide (eCO) has been assessed in healthy non-smokers with a photo acoustic spectrometer Bruel&Kjaer 1312. Unexpectedly, values were higher than those reported in literature, which were mostly obtained with electrochemical analysers. This study was aimed to compare eCO values obtained with Bruel&Kjaer 1312 and PiCO + Smokerlyzer, a largely utilized electrochemical analyser. Methods: Thirty-four healthy subjects, 15 non- smokers and 19 smokers, underwent eCO assessment with Bruel&Kjaer1312 and PiCO + Smokerlyzer during a prolonged expiration (15 seconds). Bruel&Kjaer 1312 assessed CO concentration 7 and 12 seconds after the beginning of expiration and displayed the mean value. PiCO + Smokerlyzer was utilized according to the manufacturer's recommendations. In vitro, the two devices were tested with standard concentrations of CO in nitrogen (5, 9.9, 20, and 50 ppm), and the time needed by PiCO + Smokerlyzer readings to stabilize was assessed at different gas flows. Results: Both Bruel&Kjaer 1312 and PiCO + Smokerlyzer presented very good internal consistency. The values provided were strictly correlated, but at low test concentrations, the Bruel&Kjaer 1312 readings were greater than the PiCO + Smokerlyzer, and vice versa. PiCO + Smokerlyzer overestimated the CO standard concentrations at 5 and 9.9 ppm by 20%, while Bruel&Kjaer 1312 measures were correct. PiCO + Smokerlyzer readings stabilized in 12 seconds during in vitro tests and in 15 seconds during in vivo measurements, suggesting that the values displayed corresponded to the initial phase of expiration. Conclusions: Differences between Bruel&Kjaer 1312 and PiCO + Smokerlyzer may be explained because Bruel&Kjaer 1312 measured CO levels in the middle and at the end of expiration while PiCO + Smokerlyzer assessed them in the initial part of expiration.