A stepwise approach to the interpretation of pulmonary function tests

Office-based pulmonary function testing, also known as spirometry, is a powerful tool for primary care physicians to diagnose and manage respiratory problems. An obstructive defect is indicated by a low forced expiratory volume in one second/forced vital capacity (FEV1/FVC) ratio, which is defined as less than 70% or below the fifth percentile based on data from the Third National Health and Nutrition Examination Survey (NHANES III) in adults, and less than 85% in patients five to 18 years of age. If an obstructive defect is present, the physician should determine if the disease is reversible based on the increase in FEV1 or FVC after bronchodilator treatment (i.e., increase of more than 12% in patients five to 18 years of age, or more than 12% and more than 200 mL in adults). Asthma is typically reversible, whereas chronic obstructive pulmonary disease is not. A restrictive pattern is indicated by an FVC below the fifth percentile based on NHANES III data in adults, or less than 80% in patients five to 18 years of age. If a restrictive pattern is present, full pulmonary function tests with diffusing capacity of the lung for carbon monoxide testing should be ordered to confirm restrictive lung disease and form a differential diagnosis. If both the FEV1/FVC ratio and the FVC are low, the patient has a mixed defect. The severity of the abnormality is determined by the FEV1 (percentage of predicted). If pulmonary function test results are normal, but the physician still suspects exercise- or allergen-induced asthma, bronchoprovocation (e.g., methacholine challenge, mannitol inhalation challenge, exercise testing) should be considered.