Sputum induction: effect of nebulizer output and inhalation time on cell counts and fluid-phase measures

Background A knowledge of the factors that can affect induced sputum results is essential in order to standardize the procedure. Objective We investigated the influence of nebulizer output on sputum cell counts and fluid phase measurements at increasing times of sputum induction. Methods Eighteen adults with stable asthma inhaled an aerosol of 3% hypertonic saline to induce sputum after 7, 14 and 21 min on 2 days separated by 48 h. On one day, in random order, the ultrasonic nebulizer used had a relatively low output of 0.87 mL/min (particle size 5.58 µm mass median aerodynamic diameter, MMAD) and, on the other, a higher output of 1.90 mL/min (particle size 4.14 µm MMAD). The sputum was selected from each expectorate and examined blind to the induction procedures. Results With both nebulizers, the 14‐ and 21‐min samples were lower in weight, neutrophils, eosinophils, eosinophil cationic protein (ECP) and interleukin (IL)‐8 and higher in macrophages. The higher output nebulizer induced sputum with higher cell viability and lower ECP and IL‐8. Conclusion The results identify that the volume of hypertonic saline inhaled in sputum induction influences the fluid‐phase measurements. The duration of induction does alter the cell counts and suggests that the later expectorated sputum samples originate from more peripheral airways. The results draw attention to the need to standardize the volume and time of nebulization to accurately interpret and compare results.