The impact of disease severity on EQ-5D and SF-6D utility discrepancies in chronic heart failure.

OBJECTIVES: To compare EQ-5D and SF-6D utilities across groups of chronic heart failure (CHF) patients with varying levels of disease severity.
METHODS: A consecutive sample (N = 251) of CHF patients undergoing elective cardiac surgery were surveyed. Disease severity was proxied via a self-assessment scale, the EQ-VAS and the Duke Activity Status Index (DASI); however, validity was demonstrated only by the latter. Association and level of agreement between instruments in DASI-based severity groups were estimated with Pearson's r and the intraclass correlation coefficient (ICC), respectively. Paired-samples t test was used to identify significant differences. In a linear regression model, the DASI was used as an anchor of disease severity to identify a potential "crossover" point between EQ-5D and SF-6D utilities.
RESULTS: EQ-5D and SF-6D strongly correlated over the entire sample (r = 0.647, P < 0.001); however, their agreement was moderate (ICC = 0.484, P < 0.001). In the less severe DASI groups (i.e. higher functional capacity) EQ-5D was significantly higher than SF-6D (P < 0.001) and differences constituted minimally important differences (MIDs). Contrarily, in the more severe groups SF-6D was predominantly higher than EQ-5D. The regression model indicated a utility crossover point at 0.722 and predicted that individuals with a utility score less than this would score higher on the SF-6D than on the EQ-5D, and vice versa. The DASI score at crossover was calculated at 31.94.
CONCLUSIONS: In subgroups of patients differing in CHF severity according to the DASI, mean EQ-5D and SF-6D indices differed significantly. Contrarily, in socio-demographic and clinical groups, these utility differences were not directly evident. According to the evidence, comparisons based on severity classification via a valid disease-specific external instrument may provide insight on instrument choice in cost-utility analyses.