Effect of repeated sputum induction on cell counts in normal volunteers

Background - Sputum induction is becoming more commonly used to assess airway inflammation. Since it is a relatively non-invasive procedure it may be useful for repeated measurements over a short period of time. Methods - To assess the repeatability of the method over a 24 hour period, eight healthy, non-smoking, non-atopic subjects (four men) of mean age 30 years underwent sputum induction, repeated at eight hours and 24 hours. Sputum was induced by inhalation of 3.5% saline. Absolute and differential counts of inflammatory cells were performed on the whole sputum after dilution in Hank's balanced salt solution containing 1% dithiothreitol to solubilise the mucus content of the samples. Results - There was a significant rise in the percentage of neutrophils in the eight hour sample compared with the baseline (57%, range 25-94% at eight hours compared with 28%, range 19-60%: median difference 22%) which remained high in the 24 hour sample (76%, range 37-83%: median difference 26%). The median baseline percentage of macrophages was 55% (range 26-69%) which fell to 38% (range 3-56%: median difference 22%) at eight hours and 19% (range 14-59%: median difference 25%) at 24 hours. There was no significant change in the differential counts of eosinophils, lymphocytes or columnar epithelial cells, nor in any of the absolute cell counts, at any time point. Conclusions - Sputum induction may have Limited utility in serial assessment of neutrophilic airway inflammation in normal subjects within a 24 hour period.