BackgroundPatient age and baseline Alberta Stroke Program Early CT score (ASPECTS) are both independent predictors of outcome in acute ischemic stroke patients treated with endovascular therapy (EVT). We assessed the combined effect of age and ASEPCTS on clinical outcome in acute ischemic stroke patients with LVO with and without EVT, and EVT treatment effect in different age/ASPECTS subgroups.MethodsThe HERMES collaboration pooled data of seven randomized controlled trials that tested the efficacy of EVT. Adjusted logistic regression was performed to test for multiplicative interaction of age and ASPECTS with the primary outcome (ordinal mRS) and secondary outcomes (mRS 0–2/0–1/0–3) in the EVT and control arms. Patients were then stratified by age (<75 vs ≥ 75 years) and ASPECTS (0–5/6–7/8–10), and adjusted effect-size estimates for the association of EVT were derived for the six age/ASPECTS subgroups.Results1735 patients were included in the analysis. There was no multiplicative interaction between age and ASPECTS on clinical outcomes. In the exploratory subgroup analysis, we found a nominally negative point estimate for the association of EVT with clinical outcome in the ASPECTS 0–5/age ≥75, subgroup (acOR 0.36, 95% CI 0.07 to 1.89). The point estimate for moderate outcome (mRS0-3) nominally favored EVT (aOR 1.24, 95% CI 0.16 to 9.84). In all other subgroups, effect size-estimates consistently favored EVT.ConclusionThere was no multiplicative interaction of age and ASPECTS on clinical outcomes in EVT or control arm patients. Outcomes in patients ≥75 years with ASPECTS 0–5 were poor, irrespective of treatment. Further investigation to define the role of EVT and range of acceptable outcomes in this subgroup is warranted.