The effect of filtration on absolute and differential cell counts in fluid obtained at bronchoalveolar lavage

We have studied the effects of filtering mucus from bronchoalveolar lavage (BAL) fluid on its cellular contents. We examined BAL fluid from 18 patients without excessive endobronchial mucus, and from three patients with bronchiectasis. In the non-bronchiectatic subjects there was a fall of 18% in mean absolute cell count from 2.66 +/- SD 1.9 x 10(9) l-1 to 2.18 +/- 1.4 x 10(9) l-1 (P less than 0.05) as a result of filtration. This was due to a selective loss of macrophages and there were no significant changes in the absolute numbers of either lymphocytes or neutrophils. The differential macrophage count fell from a mean of 82 +/- 14% to 76 +/- 16%, while the mean differential lymphocyte count rose from 14 +/- 7% to 19 +/- 9% and the mean differential neutrophil count increased slightly from 4 +/- 4% to 5 +/- 4%. In the three patients with bronchiectasis the absolute cell counts were grossly raised, due almost entirely to an excess of neutrophils. In these patients, filtration reduced the absolute macrophage count by 61% and the absolute neutrophil count by 27%. Overall, change in absolute cell count correlated significantly with mucus volume (r = 0.76, P less than 0.05). Removal of mucus by filtration affects absolute cell count and may have a selective effect on cell numbers. If filtration of BAL fluid is unavoidable, its effect on cell counts should be clearly established if data from different centres are to be meaningfully compared.