Induced sputum: time from expectoration to processing

One of the limitations in the use of induced sputum to measure indices of airway inflammation is the perceived need to process the sample within 2 h. Therefore, the authors investigated whether the processing of induced sputum could be delayed. Induced sputum samples obtained from asthmatic subjects (n=30) were examined. Each sample was stored at 4 degrees C. A portion was selected and processed within 2 h and the remaining expectorate (sputum plus saliva) was refrigerated. Later an equal amount was selected and processed at either 9 (n=15) or 18 (n=15) h. The sputum was examined for cell counts and viability, fluid-phase eosinophil cationic protein (ECP), interleukin-8 (IL-8) and fibrinogen. Repeatability of measurements was assessed by the interclass correlation coefficient (ICC). Measurements obtained at 9 h did not differ from those made at 2 h and the repeatability was excellent (ICC 0.88-0.99). However, by 18 h the median cell viability was reduced from 65.0% to 43.0% and the ICC was generally lower: 0.10 for total cell count, 0.24 for viability, 0.60 for neutrophils, 0.90 for eosinophils, 0.56 for macrophages, 0.76 for ECP, 0.82 for IL-8 and 0.84 for fibrinogen. The results indicate that when induced sputum from subjects with asthma is kept at 4 degrees C, examination of cell counts can be delayed for < or = 9 h and for the fluid-phase indices measured for < or = 18 h. Further investigation of this issue is required for spontaneous sputum, other airway diseases and other inflammatory markers.