Minimally Detectable Change and Minimal Clinically Important Difference of a Decline in Sentence Intelligibility and Speaking Rate for Individuals With Amyotrophic Lateral Sclerosis

Purpose

The purpose of this study was to determine the minimally detectable change (MDC) and minimal clinically important difference (MCID) of a decline in speech sentence intelligibility and speaking rate for individuals with amyotrophic lateral sclerosis (ALS). We also examined how the MDC and MCID vary across severities of dysarthria.

Method

One-hundred forty-seven patients with ALS and 49 healthy control subjects were selected from a larger, longitudinal study of bulbar decline in ALS, resulting in a total of 650 observations. Intelligibility and speaking rate in words per minute (WPM) were calculated using the Sentence Intelligibility Test (Yorkston, Beukelman, & Hakel, 2007), and the ALS Functional Rating Scale–Revised (Cedarbaum et al., 1999) was administered to capture patient perception of motor impairment. The MDC at the 95% confidence level was estimated using the following formula: MDC₉₅ = 1.96 × √2 × SEM. For estimation of the MCID, receiver operating characteristic curves were generated, and area under the curve and optimal thresholds to maximize sensitivity and specificity were calculated.

Results

The MDC for sentence intelligibility was 12.07%, and the MCID was 1.43%. The MDC for speaking rate was 36.57 WPM, and the MCID was 8.80 WPM. Both MDC and MCID estimates varied with severity of dysarthria.

Conclusions

The findings suggest that declines greater than 12% sentence intelligibility and 37 WPM are required to be outside measurement error and that these estimates vary widely across dysarthria severities. The MDC and MCID metrics used in this study to detect real and clinically relevant change should be estimated for other measures of speech outcomes in intervention research.

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