To differentiate between obstructive and restrictive lung disease using pulmonary function tests (PFTs), the key parameters to assess are the FEV1 (forced expiratory volume in 1 second), FVC (forced vital capacity), and the FEV1/FVC ratio. In obstructive lung disease, such as COPD, the hallmark is a reduced FEV1/FVC ratio (typically below 0.7 post-bronchodilator), reflecting airflow limitation due to airway obstruction. The FEV1 is reduced more than the FVC, leading to this decreased ratio. Conversely, in restrictive lung disease, both FEV1 and FVC are reduced proportionally, so the FEV1/FVC ratio is normal or even increased because the limitation is due to reduced lung volumes rather than airway obstruction NICE NG115.
Additionally, total lung capacity (TLC) measurement is important: TLC is normal or increased in obstructive disease but reduced in restrictive disease, confirming restriction. Transfer factor for carbon monoxide (TLCO) can also aid differentiation; it is often reduced in restrictive diseases involving parenchymal damage but may be normal or mildly reduced in pure obstructive disease NICE NG115.
Reversibility testing with bronchodilators is more relevant to distinguishing asthma from COPD but does not reliably differentiate obstructive from restrictive patterns. Serial peak flow measurements and symptom history complement PFT interpretation but are not definitive for this differentiation NICE NG115.
From a stepwise interpretation perspective, as outlined in recent literature, the initial focus is on the FEV1/FVC ratio to identify obstruction. If obstruction is absent but lung volumes are reduced, restriction is suspected. Confirmatory lung volume measurements and TLCO testing are recommended to clarify the diagnosis Johnson & Theurer 2014.