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The aim of this study was to examine whether significant others can provide useful proxy information on the health-related quality of life (QL) of cancer patients. We examined the level and pattern of agreement between patient and proxy ratings of the EORTC QLQ-C30, the reliability and validity of both types of information, and the influence of several factors on the extent of agreement. QL ratings were obtained for 307 and 224 patient-proxy pairs (at baseline and follow-up, respectively). Agreement was moderate to good (ICC = 0.42 to 0.79). Multitrait-multimethod analysis showed good convergence and discrimination of specific QL domains. Comparison of mean scores revealed a small but systematic bias between patient and proxy ratings. The maximum level of disagreement was found at intermediate levels of QL, with smaller discrepancies noted for patients with either a relatively poor or good QL. Both patient and proxy QL ratings were reliable and responsive to changes over time. Several characteristics of the patients and their significant others were found to be associated with the level of agreement, but explained less than 15% of the variance in patient-proxy differences. In conclusion, the present findings lend support to the viability of employing significant others as proxy respondents of cancer patients' quality of life where this is necessary.

Objectives: The use of self-report questionnaires for the assessment of health-related quality of life (HRQOL) is increasingly common in clinical research. This method of data collection may be less suitable for patient groups who suffer from cognitive impairment, however, such as patients with brain cancer. In such cases, one can consider employing the patients' significant others as proxy raters of the patients' health-related quality of life. The authors examined the response agreement between patients with brain cancer and their significant others on a health-related quality of life instrument commonly used in cancer clinical trials, the EORTC QLQ-C30, and on a brain cancer-specific questionnaire module, the QLQ-BCM., Methods: The study sample consisted of 103 pairs of patients, with either recently diagnosed or recurrent brain cancer, and their significant others (75% spouses, 22% relatives, and 3% friends). Patients and proxies independently completed the EORTC QLQ-C30 and the QLQ-BCM at three different times., Results: Approximately 60% of the patient and proxy scores were in exact agreement, with more than 90% of scores being within one response category of each other. For most HRQOL dimensions assessed, moderate to good agreement was found. Statistically significant differences in mean scores were noted for several dimensions, with proxies tending to rate the patients as having a lower quality of life than the patients themselves. With the exception of fatigue ratings, this response bias was of a limited magnitude. Less agreement and a more pronounced response bias was observed for the more impaired patients, and particularly for patients exhibiting mental confusion. This finding was confirmed by longitudinal analyses, which indicated lower levels of patient-proxy agreement at follow-up for those patients whose physical or neurologic condition had deteriorated over time., Conclusions: In general, patients and their significant others provide similar ratings of the patients' quality of life. Lower levels of agreement and more biased ratings can be expected among those patients for whom the need for proxies is most salient. It is argued, however, that discrepancies between patient-proxy ratings should not be interpreted, a priori, as evidence of the inaccuracy or biased nature of proxy-generated data. Future studies are needed to examine the relative validity and reliability of patient-versus proxy-generated health-related quality of life scores.