Exposing the Roots of Restriction: When the Transfer Coefficient Makes the Difference

D'Cruz et al discuss a case study of a 39-year-old woman with chronic juvenile idiopathic arthritis who presented with dyspnea that progressed over 5 months. The patient had a history of pulmonary embolism and was assessed for recurrent thromboembolic disease. Pulmonary function tests revealed moderately severe restriction with a marked decrease in inspiratory capacity and a supranormal transfer coefficient for carbon monoxide. They explore the relationship between capillary and alveolar volumes and how it affects carbon monoxide. They also discuss the interpretation of KCO in the context of VA measured as part of the diffusing capacity determination. The patient was diagnosed with shrinking lung syndrome and responded well to immunomodulation therapy. They emphasizes the importance of considering carbon monoxide and alveolar volume in the evaluation of pulmonary function and the diagnosis of restrictive lung diseases.