Concentration of bronchoalveolar lavage fluid by ultrafiltration: evidence of differential protein loss and functional inactivation of proteinase inhibitors.

Bronchoalveolar lavage samples were concentrated using positive-pressure ultrafiltration. The starting material, concentrates, and eluates were assayed for immunoglobulin A (IgA), albumin (Alb), alpha 1-proteinase inhibitor (alpha 1-PI), antileukoprotease (ALP), and total leukocyte elastase inhibitory capacity (LEIC). No enzyme inhibitory capacity or protein was detected in membrane eluates, confirming the selectivity of the membrane used (Mr cutoff 2000 or 500). However, the concentrated lavages showed a generated loss of protein. The proportion of each protein recovered using the 500 Mr cutoff membrane was: IgA, 50.6% (+/- 15%); albumin, 43% (+/- 8.4); alpha 1-PI, 53.6 (+/- 17.3); ALP, 43% (+/- 2.1); and LEIC, 18.4% (+/- 2.6). Similar results were obtained with the 200 Mr cutoff membrane. The alpha 1-PI/Alb and the IgA/alb ratios were higher (2P less than 0.05) in the concentrates than in the starting material, suggesting differential protein loss. Protein losses were due to binding to the membrane since the wash with saline solution improved recoveries: IgA, 80%; Alb, 56%; alpha 1-PI, 64%; ALP, 66%; LEIC, 29%. Concentration of bronchoalveolar lavage fluids therefore resulted in substantial differential losses in elastase inhibitory capacity and protein concentrations, suggesting analysis of these fluids should be performed on unconcentrated samples.