An increased concentration of a surfactant protein in the blood circulation. Pulmonary surfactant is a highly surface-active mixture of proteins and lipids that is synthesized and secreted onto the alveoli by type II epithelial cells. The protein part of surfactant constitutes of four types of surfactant proteins (SP), SP-A, SP-B, SP-C and SP-D. SP-A and SP-D are hydrophilic proteins that regulate surfactant metabolism and have immunologic functions. These two proteins are detectable in the bloodstream and an elevated level may reflect idiopathic pulmonary fibrosis. [from
HPO]