Background: The modified Rankin scale (mRS) at 3 months is established as the primary outcome measure in clinical stroke trials. Traditionally, the mRS is assessed through an unstructured face-to-face interview. This approach can be labor-intensive and lead to suboptimal inter-rater reliability. Recently, the Covid-19 pandemic made face-to-face contact even more challenging. To address these issues, we developed and validated a structured German-language questionnaire for mRS testing by telephone.

Methods: In this prospective cohort study, we compared the mRS testing results of the standard face-to-face interview with those obtained in a structured interview by telephone using Cohen’s Kappa.

Results: At our tertiary care stroke center, we included 108 patients who underwent both assessments. In 80.6% of cases (87/108) face-to-face and telephone interview reached identical scores. Linear weighted Kappa was 0.82 (p < 0.001). Unweighted Kappa for dichotomized mRS between fair (0–2) and poor (3–6) functional outcome was κ = 0.97 (p < 0.001).

Conclusions: Our study validates the use of the German-language structured telephone interview as a reliable instrument for the use in clinical trials. We encourage others to utilize the questionnaire. It is available as an Appendix (Additional file 1) to this publication.