To the Editor:
In their report on the Solitaire with the Intention for Thrombectomy as Primary Endovascular Treatment (SWIFT PRIME) trial, Saver et al. provide additional evidence on the effectiveness of endovascular therapy for acute ischemic stroke. In phase 3 stroke trials, the functional outcome on the modified Rankin scale is typically regarded as an important and valid measure. In the SWIFT PRIME trial, the effect of treatment on this functional outcome was impressively large. However, the blinding of this partly subjective assessment is crucial to minimizing bias. In this study, patients and their proxies were aware of the treatment they received, and the blind may easily have been broken. In the Multicenter Randomized Clinical Trial of Endovascular Treatment for Acute Ischemic Stroke in the Netherlands (MR CLEAN), we created masked structured reports, which were assessed by an independent committee. In the SWIFT PRIME trial, local investigators were responsible for the assessment of outcomes, even if they were not otherwise involved with the trial. This factor may have influenced the score on the modified Rankin scale.
In this otherwise very well conducted trial, the treatment effect on secondary outcomes pointed in the desired direction, which corroborates the observed effect on the primary outcome. Nevertheless, we have to remain aware that any outcome assessment that is not properly blinded, particularly in stroke trials, may lead to an overestimation of the treatment effect.

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Journal New England Journal of Medicine
Berkhemer, O.A, van Zwam, W.H, & Dippel, D.W.J. (2015). Stent-retriever thrombectomy for stroke: To the editor. New England Journal of Medicine, 373(11), 1076–1078. doi:10.1056/NEJMc1508744