Safety of sputum induction.

We have read with great interest the article by Castagnaro et al (October 1999),1 reporting a significant drop in oxygen saturation in the absence of an equivalent drop in FEV1 during sputum induction. This, however, has not been our experience. Although we agree that FEV1 may fall in asthmatics, COPD patients,2 and normal volunteers,3 we have not seen significant changes in oxygen saturation. We have found induced sputum to be a safe procedure in normal subjects, stable and exacerbated COPD patients, as well as in cystic fibrosis patients, using both lowand high-output nebulizers with no changes in oxygen saturation of this magnitude. Our mean fall in oxygen saturation was only 0.6%, even in COPD patients with very low FEV1. Although, as the authors point out, there have been no studies specifically examining changes in oxygen saturation during sputum induction, one article4 found only a small (0.63%), although statistically significant, fall in oxygen saturation. The authors describe a recent editorial by Pavord et al,5 suggesting that induced sputum may be an inflammatory stimulus and that total saline solution load may be one mechanism for this. It is likely that this explains the change in inflammatory cells seen in induced sputum 24 h after sputum induction.6 However, it is unlikely that hypertonic saline solution could cause clinically significant bronchoconstriction via such a pathway, as this occurs in some subjects within minutes. It is more likely that hypertonic saline solution acts as a direct bronchial irritant, thus provoking bronchoconstriction. In conclusion, we feel this article highlights important issues regarding the safety of sputum induction and the possible proinflammatory nature of hypertonic saline solution. However, the changes in oxygen saturation are hard to understand and are not consistent with our experience or other published studies.