The Ziegler Test Is Reliable and Valid for Measuring Freezing of Gait in People With Parkinson Disease

OBJECTIVE: The purpose of this study was to determine interrater and test–retest reliability of the Ziegler test to measure freezing of gait (FOG) severity in people with Parkinson disease. Secondary aims were to evaluate test validity and explore Ziegler test duration as a proxy FOG severity measure. METHODS: Physical therapists watched 36 videos of people with Parkinson disease and FOG perform the Ziegler test and rated FOG severity using the rating scale in real time. Two researchers rated 12 additional videos and repeated the ratings at least 1 week later. Interrater and test–retest reliability were calculated using intraclass correlation coefficients (ICCs). Bland–Altman plots were used to visualize agreement between the researchers for test–retest reliability. Correlations between the Ziegler scores, Ziegler test duration, and percentage of time frozen (based on video annotations) were determined using Pearson r. RESULTS: Twenty-four physical therapists participated. Overall, the Ziegler test showed good interrater (ICC(2,1) = 0.80; 95% CI = 0.65–0.92) and excellent test–retest (ICC(3,1) = 0.91; 95% CI = 0.82–0.96) reliability when used to measure FOG. It was also a valid measure, with a high correlation (r = 0.72) between the scores and percentage of time frozen. Ziegler test duration was moderately correlated (r = 0.67) with percentage of time frozen and may be considered a proxy FOG severity measure. CONCLUSION: The Ziegler test is a reliable and valid tool to measure FOG when used by physical therapists in real time. Ziegler test duration may be used as a proxy for measuring FOG severity. IMPACT: Despite FOG being a significant contributor to falls and poor mobility in people with Parkinson disease, current tools to assess FOG are either not suitably responsive or too resource intensive for use in clinical settings. The Ziegler test is a reliable and valid measure of FOG, suitable for clinical use, and may be used by physical therapists regardless of their level of clinical experience.