Objective To study how the choice of national EQ-5D tariff may affect utility and incremental quality-adjusted life-year (QALY) estimates.

Methods South Swedish rheumatoid arthritis patients in an observational study, starting and continuing anti-tumour necrosis factor (TNF) monotherapy (n=54) or anti-TNF plus methotrexate (n=215) for 1 year during May 2002 to April 2009, were included. EQ-5D questionnaires were completed at baseline, 3, 6 and 12 months. Utilities and accumulated QALY were compared using the UK, US and Danish EQ-5D tariffs. Utilities for all 243 possible EQ-5D health states were also compared.

Results US utilities were generally higher than UK, with Danish falling in between. A substantial 1-year mean utility improvement was seen in both study groups using all tariffs (UK 0.28 vs 0.29; US 0.18 vs 0.19; Danish 0.20 vs 0.22). Adjusting for baseline differences between groups, the incremental QALY gain of combined treatment was 0.09 using the UK tariff, while 0.06 according to both US and Danish tariffs. Inter-tariff disagreement in utility and accumulated QALY varied irregularly across the range of utilities.

Conclusions Applying different national EQ-5D tariffs to the same data may result in substantially different incremental QALY estimates, crucial knowledge when interpreting cost-utility analyses. Studies using different tariffs cannot be directly compared.