Modeling quality of life in cancer patients as a unidimensional construct.

Quality of life (QoL) in cancer patients has almost always been assessed as a multidimensional construct with subdomains including physical, emotional, social, cognitive, global, and specific symptoms. The assumed existence of multiple, and sometimes orthogonal, subdomains has prevented QoL from being defined consistently. Using an item response theory approach, this study examined the feasibility of modeling QoL as a unidimensional construct. The study sample consisted of 366 cancer patients who each responded to three QoL questionnaires: the EORTC QLQ-C30, the COOP/ WONCA, and the HI-QOL. The items from these questionnaires were pooled and examined for the information each conveyed with respect to a unidimensional QoL construct. Twenty-two items were found to perform well, suggesting the possibility of modeling QoL as a unidimensional construct. Unidimensional QoL appears to be defined by items spanning the major subdomains: global, social, emotional, physical, role, fatigue, and the ability to engage in past activities. The cognitive subdomain did not fit the unidimensional measure.