Presentation (CTI)

OBJECTIVES: Model structure variability across health technology appraisals (HTAs) hinders comparability and introduces inefficiencies in decision-making. This study examines economic model structure variability in NICE HTAs for chronic asthma and rare thrombotic microangiopathies (TMAs), including atypical haemolytic uraemic syndrome (aHUS) and acquired thrombotic thrombocytopenic purpura (aTTP). It also explores whether frequent structural variation supports the need for disease-specific reference models.
METHODS: A targeted review of publicly available NICE Single Technology Appraisals (STAs) and Highly Specialised Technologies (HSTs) was conducted for asthma and TMAs. Reports were included if they provided sufficient detail on model structure and its development or revisions. Model structure components, including health states, were extracted from Final Appraisal Determinations (FADs) and Evidence Assessment Group (EAG) reports. Structural variations and their associated rationales (e.g., new data, guideline changes, expert input, or EAG critique) were identified and inductively grouped into thematic categories. Implications of repeated model redesigns were considered in relation to the potential value of standardised reference models.
RESULTS: Asthma appraisals (n = 8) exhibited marked structural variability, with 7 (88 %) employing distinct model structures and showing little cross-appraisal consistency. Early evaluations of inhaled corticosteroids used simple Markov or decision-tree models, while later biologics appraisals adopted more complex models incorporating treatment response, asthma control, and exacerbation events. All TMA appraisals (n=3) used distinct models differing in their treatment of acute/chronic phases, plasma exchange, and relapse. Structural changes were often driven by evolving clinical evidence, HTA scope deviations, methodological guidelines, or expert opinion. Rationales were inconsistently documented, and prior models were rarely reused. The small number of TMA appraisals limits generalisability but illustrates key issues in rare disease evaluations.
CONCLUSIONS: Frequent structural variation in asthma and TMA appraisals highlights fragmented modelling approaches, which disease-specific reference models may address by improving HTA consistency and efficiency. Feasibility and stakeholder acceptance warrant further exploration.