Background
The onset of an acute exacerbation of COPD is defined by an acute worsening of symptoms over a period of approximately 7 days (Pauwels et al, ERS, 2003, P1588). We examined the change in symptoms from the chronic to acute phase of bronchitis for their sensitivity to the development of an exacerbation.
Method
A total of 730 patients (FEV1 46 sd 19 % pred) were administered the 7-item ABSS questionnaire in stable state and subsequently during an acute infective exacerbation. Consistency of the 5 severity-graded responses to each of the 7 items were examined using Rasch analysis. Significant differences between observed item responses and the Guttman-scaling expectations of the Rasch model are tabulated. A p value <0.05 indicates that an items responses do not fit with the responses to other items.

[Table]

Responses to sputum colour had good scaling properties during an acute infective epsiode but not when stable. Change in colour of phlegm score between chronic and acute phase of bronchitis was greater than change in the other 6 items (p<0.0001).
Conclusions
Colour of sputum is independent of other symptoms of bronchitis in both the stable and acute phases of chronic bronchitis. This item has good scaling properties and may represent a marker of an exacerbation whilst other symptoms reflect changes in severity along a continuous scale.