A 62 year old man with a 60 pack year cigarette smoking history presents with a chief complaint of progressive dyspnea on exertion and a chronic productive cough over the past eight years. Significant contributing history include two episodes of being seen in urgent care in the past year for "bronchitis" and being told he should follow up with primary care. When considering the diagnosis of COPD, which of the following is the most important diagnostic parameter? A. patient report ...