Creating scenarios of the impact of COPD and their relationship to COPD Assessment Test (CAT™) scores

Abstract

Background: The COPD Assessment Test (CAT™) is a new short health status measure for routine use. New questionnaires require reference points so that users can understand the scores; descriptive scenarios are one way of doing this. A novel method of creating scenarios is described.

Methods: A Bland and Altman plot showed a consistent relationship between CAT scores and scores obtained with the St George's Respiratory Questionnaire for COPD (SGRQ-C) permitting a direct mapping process between CAT and SGRQ items. The severity associated with each CAT item was calculated using a probabilistic model and expressed in logits (log odds of a patient of given severity affirming that item 50% of the time). Severity estimates for SGRQ-C items in logits were also available, allowing direct comparisons with CAT items. CAT scores were categorised into Low, Medium, High and Very High Impact. SGRQ items of corresponding severity were used to create scenarios associated with each category.

Results: Each CAT category was associated with a scenario comprising 12 to 16 SGRQ-C items. A severity 'ladder' associating CAT scores with exemplar health status effects was also created. Items associated with 'Low' and 'Medium' Impact appeared to be subjectively quite severe in terms of their effect on daily life.

Conclusions: These scenarios provide users of the CAT with a good sense of the health impact associated with different scores. More generally they provide a surprising insight into the severity of the effects of COPD, even in patients with apparently mild-moderate health status impact.

Figures

Figure 1

Cumulative frequency distribution of CAT scores.

Figure 2

Mapping CAT scores to SGRQ items. See text for full explanation.

Figure 3

Bland and Altman plot of SGRQ and adjCAT scores. CAT scores were converted to 0 to 100% (adjCAT) to match SGRQ scores. The X axis is the mean of the SGRQ and adjCAT scores; the y axis is SGRQ-adjCAT score. The correlation for a linear regression was r = 0.16, p = 0.005.