Effect of repeated sputum induction on cell counts in normal volunteers

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.

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.

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 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.

Sputum induction may have limited utility in serial assessment of neutrophilic airway inflammation in normal subjects within a 24 hour period.