Counterfactual Clinical Prediction Models Could help to Infer Individualised Treatment Effects in Randomised Controlled Trials - an Illustration with the International Stroke Trial.

OBJECTIVE: Causal treatment effects are estimated at the population level in randomised controlled trials (RCTs), whilst clinical decision is often to be made at the individual level in practice. We aim to show how clinical prediction models used under a counterfactual framework may help to infer individualised treatment effects. STUDY DESIGN AND SETTING: As an illustrative example, we reanalyse the International Stroke Trial. This large, multi-centre trial enrolled 19,435 adult patients with suspected acute ischaemic stroke from 36 countries, and reported a modest average benefit of Aspirin (versus no Aspirin) on a composite outcome of death or dependency at 6 months. We derive and validate multivariable logistic regression models that predict the patient counterfactual risks of outcome with and without Aspirin, conditionally on 23 predictors. RESULTS: The counterfactual prediction models display good performance in terms of calibration and discrimination (validation c-statistics: 0.798 and 0.794). Comparing the counterfactual predicted risks on an absolute difference scale, we show that Aspirin - despite an average benefit - may increase the risk of death or dependency at 6 months (compared to the control) in a quarter of stroke patients. CONCLUSIONS: Counterfactual prediction models could help researchers and clinicians (i) infer individualised treatment effects and (ii) better target patients who may benefit from treatments.