Minimum Detectable Measurement Difference for Health-Related Quality of Life Measures Varies With Age and Disability in Adult Spinal Deformity: Implications for Calculating Minimal Clinically Important Difference

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Abstract

Study Design.

Retrospective cohort.

Objective.

To investigate the minimum detectable measurement difference (MDMD) in the Scoliosis Research Society-22r (SRS-22r) outcomes instrument in adult spinal deformity (ASD) and to evaluate the effect of baseline data on measurable difference.

Summary of Background Data.

The minimum clinically important difference (MCID) is the smallest, clinically relevant change observed and has been proposed for the SRS-22r instrument in ASD as 0.4. The MCID must be greater than the MDMD to be useful. The MDMD for the SRS-22r has not been calculated, nor have the effect of patient baseline values on MDMD.

Methods.

A prospective observation cohort was queried for patients treated both operatively and nonoperatively for ASD. Patients with baseline and 1-year, 2-year follow-up SRS-22r data were included in the analysis. The MDMD was calculated using classical test theory and item-response theory methods. Effect size and standardized response means were calculated. The effect of baseline data values was evaluated for MDMD.

Results.

A total 839 Patients were eligible for cohort inclusion with 428 (51%) eligible for analysis with complete data. MDMD for Pain (0.6) and Self-Image (0.5) were greater than 0.4. MDMD varied with age (highest for the youngest patients) and with disability (highest for SF-36 Physical Component Summary <28.6). MDMD was less than 0.4 for Activity (0.3), Mental Health (0.3), and Total Score (0.2). Gender and mental health did not affect MDMD for the SRS-22r instrument.

Conclusion.

An MCID of 0.4 for the SRS-22r total score and domain scores may not be an appropriate value as the calculated MDMD is greater than 0.4 for both the Pain and Self-Image subscores. The MDMD for the SRS-22r instrument varied with age and baseline disability, making the assessment of clinically significant change more difficult using this tool. The MCID must be considered in the setting of the MDMD for instruments used to assess outcomes in ASD.

Conclusion.

Level of Evidence: 3

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