BACKGROUND: The recent Movement Disorders Society (MDS)-progressive supranuclear palsy (PSP) diagnostic criteria conceptualized three clinical diagnostic certainty levels: "suggestive of PSP" for sensitive early diagnosis based on subtle clinical signs, "possible PSP" balancing sensitivity and specificity, and "probable PSP" highly specific for PSP pathology.

OBJECTIVE: The aim of this study was to prospectively validate the criteria against long-term clinical follow-up and characterize the diagnostic certainty increase over time.

METHODS: Patients with "possible PSP" or "suggestive of PSP" diagnosis and clinical follow-up were recruited in two German multicenter longitudinal observational studies (ProPSP and DescribePSP). The cumulative percentage of patients longitudinally increasing diagnostic certainty was assessed over up to 2.5 years of follow-up. The sample size per arm required to detect 30% attenuated rate in diagnostic certainty increase in trials was estimated over multiple time intervals.

RESULTS: Of 254 patients with available longitudinal data, 61 patients had low diagnostic certainty at baseline (48 suggestive of PSP, 13 possible PSP) and multiple clinical visits (median: 3, range: 2-4). The cumulative percentage of patients increasing diagnostic certainty progressed with follow-up duration (30.4% at 6 months, 51.7% at 1 year, 80.4% at 2.5 years). The sample size required to detect 30% reduction in diagnostic certainty increase rate within 1 year was 163, slightly smaller than that required using the PSP rating scale.

CONCLUSIONS: Most "suggestive of PSP" patients increased diagnostic certainty upon longitudinal follow-up, providing the first prospective multicenter validation of MDS-PSP diagnostic criteria. Our data support the design of trials tailored for these early-stage patients, suggesting the PSP rating scale and the diagnostic certainty increase rate as potential endpoint measures. © 2025 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.