To examine the test-retest reliability of the Functional Assessment of Cancer Therapy - 8 Dimension (FACT-8D) for the first time, and to conduct a head-to-head comparison of the distribution properties and validity between the FACT-8D and EQ-5D-5L in Colorectal Cancer (CRC) Patients. We conducted a longitudinal study on Chinese CRC patients, employing Functional Assessment of Cancer Therapy-General (FACT-G) and EQ-5D-5L at baseline, and FACT-G during follow-up (2-7 days from baseline). Utility scores for FACT-8D were derived from all available value sets (Australia, Canada and USA), while EQ-5D-5L scores were obtained from corresponding value sets for various countries. We assessed convergent validity using pairwise polychoric correlations between the FACT-8D and EQ-5D-5L; known-groups validity by discriminating participants' clinical characteristics, and effect size (ES) was tested; test-retest reliability for FACT-8D using kappa and weighted Kappa for choice consistency, and intraclass correlation coefficient (ICC) and Bland-Altman method for utility consistency. Among the 287 patients with CRC at baseline, 131 were included in the retest analysis. The utility scores of FACT-8D were highly positively correlated with EQ-5D-5L across various country value sets (r = 0.65-0.77), and most of the dimensions of FACT-8D and EQ-5D-5L were positively correlated. EQ-5D-5L failed to discriminate known-groups in cancer stage across all value sets, whereas both were significant in FACT-8D (ES = 0.35-0.48, ES = 0.38-0.52). FACT-8D showed good test-retest reliability (Cohen's weighted Kappa = 0.494-0.722, ICC = 0.748-0.786). The FACT-8D can be used as a valid and reliable instrument for clinical evaluation of patients with CRC, outperforming EQ-5D-5L in differentiating clinical subgroups and showing promise for cancer practice and research.
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