Clinical phenotyping may be complicated for several reasons. First, the presentation of some clinical phenotypes may change due to the effect of therapy and/or the natural course of the disease. Secondly, although a COPD phenotype describes differences between individuals with COPD, a given patient can have more than one clinical phenotype. Finally, two prevalent diseases can coexist (e.g. COPD and obstructive sleep apnoea, or COPD and asthma).