Lung secretion sol-phase proteins: comparison of sputum with secretions obtained by direct sampling

The protein content of tracheal secretions and of bronchial lavage and bronchoalveolar lavage fluid was compared with that of sputum in 33 patients who underwent fibreoptic bronchoscopy. The secretion-to-serum concentration ratios for albumin, alpha 1-antitrypsin, alpha 1-antichymotrypsin, and immunoglobulin A (IgA) fell progressively as samples were obtained from further down the bronchial tree, probably reflecting greater sample dilution. The secretion-to-serum protein ratios when standardised for the corresponding albumin ratio were similar in all secretions studied. In particular, the IgA ratios were about eight times those of albumin and alpha 1-antichymotrypsin ratios were about twice those of albumin, suggesting similar degrees of "local production" of these proteins in all secretions studied. Some patients showed considerable differences in IgA ratios between sputum and bronchial lavage fluid. The significance of these differences is not clear.