Comparative analysis of diffusion rates in budget impact analyses submitted to the Brazilian Supplementary Health System

Introduction: Budget impact analysis (BIA) is a mandatory requirement for the incorporation of healthcare technologies into the Brazilian National Supplementary Health Agency Coverage List and relies heavily on assumptions regarding technology diffusion. This study compared diffusion rates proposed by applicants with those adopted in Critical Appraisal Reports (CARs) and assessed the impact of these differences on budget impact estimates and coverage decisions. Methods: A retrospective documentary review using a mixed-methods approach was conducted. Technical assessment reports of drugs and immunobiological products submitted through the FormRol process between 2021 and 2025 were reviewed. Quantitative analyses evaluated differences between applicant-proposed and regulator-adopted diffusion rates, while qualitative thematic analysis examined the justifications reported in the CARs. Results: Eighty-five technical assessments were included. Changes to the applicant-proposed diffusion rate were identified in 52 (61.2%), comprising increases in 25 (29.4%) and reductions in 27 (31.8%). An asymmetric pattern emerged: among the 33 UATs with an applicant-projected negative budget impact (cost-saving), 16 (48.5%) had the diffusion rate reduced and 3 (9.1%) increased; among the 52 with a projected positive budget impact, 22 (42.3%) had the rate increased and 11 (21.2%) reduced. Among UATs with a positive projected impact and an increased rate, the mean absolute increase in year 5 was 37 percentage points. Seven of the 33 submissions projecting savings (21.2%) were reclassified as cost-increasing after reanalysis. Thematic analysis identified 16 categories of justification for modifying diffusion rates, the most frequent being therapeutic gap, present in 14 reports (26.9%). Conclusions: Divergences in diffusion rate assumptions were frequent and followed an asymmetric pattern according to the direction of the applicant-projected budget impact, with a substantial effect on budget estimates, including reversal of the direction of the result in a subset of submissions. Justifications centered on judgements about the incorporation scenario and on clinical considerations, and explicit justification was frequently absent. These findings indicate opportunities for methodological improvement, including the use of real-world utilization data and the systematic reporting of the sources and assumptions used by both applicants and the regulator, to improve the consistency, transparency and predictability of budget impact analyses in the Brazilian supplementary health sector.