Interpreting score differences in the Insomnia Severity Index: using health-related outcomes to define the minimally important difference

Abstract

Objective:

To estimate the minimally important difference (MID) for the Insomnia Severity Index (ISI) by examining the association of score differences of the ISI with health-related outcomes including health-related quality of life, productivity, and fatigue.

Methods:

Data came from a randomized, placebo-controlled clinical trial evaluating the long-term efficacy of eszopiclone for primary insomnia (N = 828). Logistic regression models were used to characterize the relationship between ISI change scores (from baseline to 6 months post-treatment) and outcomes/anchors from the SF-36 Health Survey, Work Limitations Questionnaire (WLQ), and Fatigue Severity Scale (FSS). Odds ratios were derived from the regression coefficients to calculate the probability of a given outcome being associated with different ISI change scores. Convergence between anchor- and distribution-based estimates was assessed.

Results:

Higher ISI scores (indicating more severe insomnia) were significantly associated with higher probabilities of negative outcome in all models. Individuals with a 6-point score reduction in ISI scores (which corresponded to 1½ standard deviations) were 48% less likely to report ‘feeling worn out’ (SF-36) at 6 months, 46% less likely to be ‘unable to think clearly’ (WLQ), and 52% less likely to report ‘feeling fatigued’ (FSS). Similar results were found across a broad spectrum of all selected anchors.

Conclusions:

Based on results of the study, a 6-point reduction is recommended to represent a clinically meaningful improvement in individuals with primary insomnia. Research on generalizability of the recommended MID in this study to other patient populations and other type of treatment interventions is needed.

Key words: :

• Insomnia
• MID
• Minimally important difference
• Patient-reported outcomes

Transparency

Declaration of funding

This study was supported by Sepracor Inc.

Declaration of financial/other relationships

M.Y. has disclosed that she is an employee of QualityMetric, which ran the study. C.M.M. has disclosed that he is a consultant for Sepracor. K.S. has disclosed that she is an employee of Sepracor. G.V.W. has disclosed that he is a former employee of QualityMetric.

All peer reviewers receive honoraria from CMRO for their review work. Peer reviewer 1 has disclosed she has no relevant financial relationships. Peer reviewer 2 has disclosed he is a stockholder of Pfizer, Inc.

Acknowledgment

The authors have disclosed that no editorial assistance was provided during the preparation of this manuscript.