Purpose: Recent research has highlighted the importance of subgroup analysis to facilitate the use of comparative effectiveness research in shared decision making (Basu, MDM, 2009). Obtaining sufficient data for such analyses may require the use of large observational data sets, particularly where adverse events/failures are rare or occur over extended time periods. Potential pitfalls can still arise when examining small differences across subgroups. We illustrate these issues, in the context of a high-profile example, prosthesis selection for primary total hip replacement (THR). Here decision-makers require cost-effectiveness results for pre-defined age and gender groups.
Method: A Markov model of THR was populated with data from three large databases to compare the cost-effectiveness of cemented, uncemented and hybrid prostheses. Patient reported outcomes on THR are now routinely collected in England providing Generic (EQ5D) and condition specific QoL data before and after THR (n = 10,000). Data on prosthesis survival was taken from the National Joint Register (NJR) of England and Wales (n=217,000) and THR admissions data for English National Health Service Hospitals (HES) (n=457,000). Ordinary least squares regression analysis was used to report QoL following THR with each prosthesis type, for different patient subgroups, adjusting for baseline differences. Alternative model specifications were considered using measures of model fit such as AIC. Combination of data from HES and NJR allowed a semi-parametric consideration of prosthesis survival up to ten years with parametric extrapolation beyond ten years by patient subgroup.
Result: Across the age range considered (60 to 80), cemented prostheses were cheaper and offer superior survival, but hybrid prostheses provide larger gains in QoL. The regression results suggest that the relative gains in QoL for hybrid prostheses may be greater for younger patients. After inclusion of subgroup interactions cemented prostheses dominate hybrid and uncemented prostheses for eighty year olds. Hybrid prostheses are the most cost-effective alternative for sixty and seventy year olds (at λ=£20,000 per QALY the incremental net benefit for females age 70 are: uncemented, £181,000; cemented, £183,000; hybrid £184,000).
Conclusion: Large observational databases can allow crucial parameters in CEA models such as QoL and survival gains to be estimated both overall and for subgroups of high policy interest. This can help both providers and patients make more informed choices about competing alternatives.