Changing Pattern of Sputum Cell Counts During Successive Exacerbations of Chronic Obstructive Pulmonary Disease. 
Chronic Obstructive Pulmonary Disease exacerbations are associated with worsening of airway inflammation, the nature of which may be neutrophilic, eosinophilic, or both.  The primary objective was to examine the cellular nature of airway inflammation in successive COPD exacerbations in order to ascertain if they changed in individual patients.  The secondary objective was to estimate the relative risk indicating the extent to which a particular type of exacerbation changed as a function of the most recent exacerbation.  This was a retrospective survey performed on a computerised sputum cell count database of a referral respiratory service in Hamilton, Canada.  Recurrent event analyses were used to model the incidence of exacerbations and subtypes of exacerbations.  359 patients and 148 patients had sputum examined during stable condition and during exacerbations, respectively.  It was found 65 patients had sputum examined during both situations.  The exacerbations were eosinophilic in 15.9%, neutrophilic in 18%, combined in 2.6%, of unknown clinical significance in 19.6% and normal in 19.6%.  There were missing counts for 24.3% samples.  In 85.2% of patients, a different subtype of bronchitis was noted in successive exacerbations.  The relative risk of a subsequent neutrophilic or eosinophilic exacerbation was 6.24 (p = 0.02) and 2.8 (p = 0.24) when the previous exacerbation was neutrophilic or eosinophilic respectively.  This non-intervention study suggests that the cellular nature of bronchitis is largely unpredictable and needs to be examined at each COPD exacerbation This has important implications in choosing the appropriate therapy.  Future intervention studies would provide further evidence.