Neutrophils in induced sputum arise from central airways

A high neutrophil count is often found in induced sputum compared to bronchoalveolar lavage fluid (BALF). This study investigated whether such a high neutrophil count may be a response to inhaled hypertonic saline or that the airway compartment sampled by induced sputum has a higher concentration of neutrophils than BALF. Saliva and induced sputum samples were taken at 10 and 20 min following inhalation of 3% hypertonic saline from an ultrasonic nebulizer in 12 healthy nonsmoking subjects. Four days later the 12 subjects underwent bronchoscopy (six following inhalation with 3% saline). Tracheal, proximal bronchial secretions, bronchial washings and BALF samples were obtained. The neutrophil count (% total leukocytes) increased significantly in saliva at 10 and 20 min post nebulization with 3% saline, although there was no change in neutrophils in induced sputum at 10 and 20 min. There was no significant difference in neutrophil count in the subjects who inhaled 3% saline as compared to those who did not, in secretions from the trachea, proximal bronchi, bronchial washings and BALF. Neutrophil counts were significantly higher in the trachea, proximal bronchi and bronchial washings as compared to BALF (p<0.001). It is concluded that neutrophil counts in healthy subjects increase from the peripheral towards the proximal airways, in the absence of hypertonic saline-induced changes. This suggests that the relatively high neutrophil count in induced sputum arises from the proximal airways and is not a response to inhaled hypertonic saline during the procedure.