In this study we test the hypothesis that endogenous particles in exhaled air (PEx), non-invasively sampled from lower airways, are well suited for the analysis of respiratory tract lining fluid (RTLF) proteins, i.e., surfactant protein A (SP-A) and albumin. Ten healthy volunteers were included in the study and participated in two sampling sessions. Blood, exhaled breath condensate (EBC) and PEx were collected at each session. 100 L of breath were collected for each exhaled sample. Serum and exhaled samples were analyzed for SP-A using an in-house ELISA. Albumin was analyzed in exhaled samples using a commercial ELISA kit. SP-A detection rates were 100%, 21%, and 89% for PEx, EBC and serum, respectively. Albumin was detected in PEx, but not in EBC. SP-A measurements in PEx showed good repeatability with an intra-individual coefficient of variation of 13%. Both SP-A and albumin showed significant correlation to mass of PEx (r(s) = 0.93, p < 0.001 and r(s) = 0.86, p = 0.003, respectively). Sampling and analysis of PEx is a valid non-invasive method to monitor RTLF proteins sampled from the lower respiratory tract, as demonstrated here by example of SP-A and albumin analysis.
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