Individualized Treatment Rules Based on Cost-Effectiveness Criteria in Microsimulations

In cost-effectiveness analysis, treatment decisions are analysed at the population level. Combinations of treatment strategies that account for the heterogeneity of costs and effects across patients can be more cost-effective than a "one size fits all" approach. Individualized treatment rules (ITRs) assign a specific treatment to every patient based on their relevant characteristics, such that overall cost-effectiveness is optimized, but do not include feasibility or ethical considerations. We propose an approach for the design of ITRs based on simulated patient data from microsimulation models using statistical learning techniques.

We mathematically define the optimal ITR and how to measure the value of an ITR in a cost-effectiveness context. We explore least absolute shrinkage and selection operator (LASSO) regression, classification trees, and policy trees to illustrate how standard statistical learning techniques can be used to derive ITRs. We compare the strengths and limitations of these three approaches in terms of three criteria: the incremental value of the ITRs compared to optimal treatment assignment in terms of net monetary benefit (NMB), computational speed, and the interpretability of the ITRs. We propose methods to describe the impact of parameter uncertainty on the ITRs. We also explore how stochastic uncertainty can impact the ITR incremental value. We illustrate the methods by applying them to a microsimulation model for haemophilia B comparing four treatment strategies as a case study. The relevant patient characteristics in this model are the annualized bleeding rate, age, and sex.

In our case study, a simple two-layer-deep classification tree is best suited based on the three criteria. This classification tree allocates treatments depending on whether the annualized bleeding rate of a patient is above or below 30 and whether their age is above or below 51. The optimal threshold values are uncertain based on the 95% credible ranges from the probabilistic analysis: 21-46 for annualized bleeding rate and 42-56 for age. Scenarios show that stochastic uncertainty has an impact on the incremental value of the ITR.

Based on methodological considerations and the empirical findings in our case study, we expect the superiority of classification trees for the derivation of ITRs to be generalizable to other microsimulation models. This finding needs to be confirmed in future applications. Stochastic uncertainty has significant impacts on the ITRs, such that accurate representations of individual patient pathways are particularly crucial when designing ITRs. Future research could explore further empirical models and analytical approaches for ITRs or consider the translation of ITRs into the real-world decision-making context.