Obstructive lung disease is usually a differential diagnostic consideration when a patient presents with breathlessness or cough. Spirometry is the key diagnostic test used to confirm airflow obstruction particularly in the primary care setting. Airflow obstruction that completely resolves after administration of a bronchodilator, by definition, excludes a diagnosis of COPD. Evaluation of obstructive lung disease must include pulmonary function testing; bronchoreversibility testing is an adjunct in differentiating between asthma and COPD. Bronchoreversibility cannot serve as an absolute diagnostic criterion for separating asthma from COPD.