Responsiveness and construct validity of EPIC-26, AQoL-6D and SF-6D following treatment in prostate cancer

To assess construct validity and responsiveness of the Expanded Prostate Cancer Index Composite Instrument (EPIC-26) relative to the Short-Form Six-Dimension (SF-6D) and Assessment of Quality of Life 6-Dimension (AQoL-6D) in patients following treatment for prostate cancer.

Retrospective prostate cancer registry data were used. The SF-6D, AQoL-6D, and EPIC-26 were collected at baseline and one year post treatment. Analyses were based on Spearman's correlation coefficient, Bland-Altman plots and intra-class correlation coefficient, Kruskal Wallis, and Effect Size and the Standardised Response Mean for responsiveness.

The study sample was comprised of 1915 patients. Complete case analysis of 3,697 observations showed moderate evidence of convergent validity between EPIC-26 vitality/hormonal domain and AQoL-6D (r = 0.45 and 0.54) and SF-6D (r = 0.52 and 0.56) at both timepoints. Vitality/hormonal domain also showed moderate convergent validity with coping domain of AQoL-6D (r = 0.45 and 0.54) and with role (r = 0.41 and 0.49) and social function (r = 0.47 and 0.50) domains of SF-6D at both timepoints, and with independent living (r = 0.40) and mental health (r = 0.43) of AQoL-6D at one year. EPIC-26 sexual domain had moderate convergent validity with relationship domain (r = 0.42 and 0.41) of AQoL-6D at both timepoints. Both AQoL-6D and SF-6D did not discriminate between age groups and tumour stage at both timepoints but AQoL-6D discriminated between outcomes for different treatments at one year. All EPIC-26 domains discriminated between age groups and treatment at both timepoints. The EPIC-26 was more responsive than AQoL-6D and SF-6D between baseline and one year following treatment.

AQoL-6D can be used in combination with EPIC-26 in place of SF-12. Although EPIC-26 is not utility based, its popularity amongst clinicians and ability to discriminate between disease-specific characteristics and post-treatment outcomes in clinical trials makes it a candidate for use within cost-effectiveness analyses. The generic measure provides a holistic assessment of quality of life and is suitable for generating quality adjusted life years (QALYs).