Alteration of bone mineral density in cystic fibrosis adults.

responses to these items, despite the inclusion of the “unable” option, suggests that the activities queried by these items may in fact be less relevant to the study population. In a recent publication by Barley and Jones,2 Rasch analysis was used to assess the measurement properties of the AQ20 over time. Item 3 was found to be unstable due an increase in “not applicable” responses related to seasonal changes over the study period. The variable relevance of these items to subjects, rather than the inability to perform these activities, may explain the lack of correlation between “not applicable” responses and other health status measures noted by Blanco-Aparicio and Vázquez. Nonetheless, we believe that providing an “unable” response option further minimizes the potential for any misclassification, particularly among those subjects with a high degree of respiratory impairment. We agree with the final conclusion of Blanco-Aparicio and Vázquez1 that additional studies of this question are needed.