Supplementary Materials: Reference Prices Matter- Impact on Cost Analyses in Economic Evaluations

Introduction: Variability in methodology and pricing across country-specific health economic evaluation (EE) guidelines hampers cross-national transferability. Guideline updates can also impede generalizability of findings within countries over time. This study empirically illustrates how changes in reference prices impact cost analysis within (partial) EEs, using the economic burden associated with adolescent idiopathic scoliosis (AIS) in the Netherlands and the United Kingdom (UK) as an example.

Methods: Country-specific databases were used to estimate costs to compare within and
cross-country EE guidelines. Reference prices from the Dutch (2014&2024) and UK (2021-
2022&2022-2023) guidelines were considered, resource usage was held constant. Costs (€) were indexed (2022) and adjusted using Purchasing Power Parity. The societal cost per AIS patient, share attributions, and unit costs between EE guidelines were compared.

Results: Within-country comparisons indicated total societal cost increases of 18% in the
Netherlands and 1% in the UK. Due to a Dutch guideline update 12 cost items changed by
over 10%, while only four did in the updated UK guideline. The UK societal costs were 69%
higher than those in the Netherlands. Ten cost items differed by over 100%, with UK costs
being generally higher.

Conclusion: EEs cost analyses outcomes are impacted by differences in reference prices
across countries. National guideline updates substantially impact the generalizability of cost analyses within countries over time. Greater transparency in references price estimations is needed to improve transferability and generalizability. Developing a standardized (e.g., European) methodology for deriving reference prices may reduce variability and enhance consistency in international and longitudinal EEs.