We read with great interest the study by Bonella et al., in which the authors evaluated the utility of serum YKL-40 as a biomarker for pulmonary alveolar proteinosis (PAP).1 They found that YKL-40 correlated with respiratory impairment and disease outcome. The study subjects had primary autoimmune PAP, characterized by elevated GM-CSF antibodies. Less commonly, secondary PAP can occur in association with immunosuppression, haematological disorders and occupational inhalation exposure to certain dusts or fumes. GM-CSF antibodies are generally within the normal range in cases of secondary PAP, although the role of GM-CSF antibodies in secondary PAP due to occupational dust exposure remains an area of active inquiry.2