This section provides guidance on interpreting pulmonary function tests (PFTs) for medical directors and physicians responsible for interpreting PFT results. It covers the interpretation of spirometry, bronchodilator response, carbon monoxide diffusing capacity (DLCO), and lung volumes. The importance of quality assurance and comparison with reference values is emphasized, along with the need to consider clinical context and patient history. The document discusses the selection of appropriate reference equations, the interpretation of different types of ventilatory defects (obstructive, restrictive, and mixed), and the severity classification of lung function impairment. It also addresses the interpretation of bronchodilator response, including the variability in responses and the clinical significance of changes in lung function parameters. The section concludes with a summary of key points and tables for reference values and severity classification.This section provides guidance on interpreting pulmonary function tests (PFTs) for medical directors and physicians responsible for interpreting PFT results. It covers the interpretation of spirometry, bronchodilator response, carbon monoxide diffusing capacity (DLCO), and lung volumes. The importance of quality assurance and comparison with reference values is emphasized, along with the need to consider clinical context and patient history. The document discusses the selection of appropriate reference equations, the interpretation of different types of ventilatory defects (obstructive, restrictive, and mixed), and the severity classification of lung function impairment. It also addresses the interpretation of bronchodilator response, including the variability in responses and the clinical significance of changes in lung function parameters. The section concludes with a summary of key points and tables for reference values and severity classification.