[Interpretation of exhaled CO levels in studies on smoking]

Exhaled carbon monoxide (ECO) concentration provides an easy, fast and non invasive means of assessing smoking status. However, some difficulties have to be considered when interpreting ECO levels.

CO is a non-specific biomarker of tobacco exposure. Its half-life is short (2-6 hours). CO reflects the intensity of tobacco smoke inhalation. Its high affinity for haemoglobin, producing carboxyhaemoglobin (COHb), leads to tissue hypoxia. The correlation between ECO and COHb is strong and linear. Gender, pulmonary ventilation, physical exercise and time of day may induce variations in CO elimination. ECO levels are increased in the following situations: consumption of alcohol or polyol-rich sweets, lactose intolerance, haemolytic anaemia, types 1 and 2 diabetes, asthma, chronic obstructive pulmonary disease (COPD) and bronchiectasis. Airflow obstruction alters the correlation between ECO and blood COHb. Smokers and nonsmokers can be differentiated by threshold levels of ECO in the range three to ten parts per million (ppm). For patients suffering from asthma and COPD, however, the threshold levels of ECO are 10 and 11 ppm respectively.

Both false positive results and clinical disorders associated with higher production of endogenous CO have to be taken into account when blood COHb and/or ECO levels are interpreted in clinical practice.