We have read with great interest the article by Mackay et al published in CHEST (May 2016) entitled “Airway Surfactant Protein D Deficiency in Adults with Severe Asthma.” As elegantly shown, serum surfactant protein D (SP-D) concentrations were significantly lower in individuals with severe asthma as opposed to individuals with mild asthma and healthy control subjects, and SP-D levels were significantly higher in the BAL of patients with severe asthma compared with those with mild asthma. On this basis, the authors propose that serum SP-D could serve as a biomarker of the most severe forms of asthma. In their article, Mackay et al state that “To the best of our knowledge, this study is the first to investigate the relationship between asthma and SP-D concentration in patients with such severe disease.” We respectfully disagree with this statement, since we recently reported that indeed serum SP-D concentrations are significantly higher in individuals with severe asthma as opposed to healthy control subjects and individuals with mild asthma, and their levels correlate with the degree of airway obstruction. In our study, we hypothesized that serum SP-D could increase as a result of inflammation-induced permeability of the bronchial microvasculature, which allows the passage of large macromolecules, such as hydrophilic surfactant proteins, from lung to blood. However, the increased levels of surfactant proteins in serum could also be explained by an increased local synthesis of surfactant proteins induced by local inflammation.