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

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

Within-country comparisons indicated societal cost increases in the Netherlands (18 percent) and the UK (1 percent). Following a Dutch guideline update, twelve cost items changed by >10 percent, compared with four did in the UK update. UK societal costs were 69 percent higher than in the Netherlands. Ten cost items differed by >100 percent, with generally higher UK costs.

EEs cost analyses are substantially influenced by differences in reference prices across countries. National guideline updates can markedly affect the within-country comparability and generalizability of cost analyses over time. Increased transparency in methods used to derive reference prices is essential to improve transferability and interpretation of EE results. A standardized approach for estimating reference prices could reduce methodological variation, strengthen consistency across jurisdictions, and enhance comparability of international and longitudinal EEs.