The effect of adding a cognitive dimension to the EuroQol multiattribute health-status classification system

A methodological study was conducted to examine the effect of extending a frequently used simple multiattribute health-status classification system by adding a cognitive dimension. The EQ-5D questionnaire is a generic instrument to value health, developed by the EuroQol Group. The EQ-5D defines health according to five dimensions: mobility, self-care, usual activities, pain/discomfort, and anxiety/depression. We defined 18 different health states according to the standard EQ-5D classification. A second set of health states was constructed similar to the first, except for the addition of a cognitive dimension (EQ-5D+C). Valuations of both sets of health states were statistically analyzed to detect the effect of the additional dimension. The cognitive dimension generated systematically different values compared with the standard EQ-5D version, whereas the content validity improved. Both systems evoked equally reliable values. Analyses showed that a simple additive model to predict summary values for health states was not optimal for both systems. Although there is a current lack of consensus regarding the domains that are selected to represent health status, this study has shown the importance of considering the inclusion of a cognitive domain.