Supplementary Material for: Development of the Gamma-hydroxybutyrate withdrawal symptom questionnaire (GWSQ)

Introduction
Gamma-hydroxybutyrate (GHB) dependence has been associated with a potentially fulminant withdrawal syndrome. As GHB withdrawal symptoms can rapidly progress from mild to more severe, close monitoring during detoxification is important. However, a concise, GHB-specific withdrawal scale is currently lacking. The aim of this study is to compose a concise GHB-specific withdrawal scale as monitoring instrument.

Method
This study used monitoring data from 285 patients with GHB use disorder (GUD) admitted for inpatient detoxification. The six-step plan for shortening measurement scales proposed by Goetz et al. (2013) was applied. Only subjective GHB withdrawal symptoms were used. To identify the factor structure, a principal component analysis (PCA) with Direct Oblimin Rotation was performed. The pattern matrix was used to assess the factor loading per item. Item selection was based on factor loadings >0.4 and clinical relevance of the symptoms. Multiple lists were created whose psychometric qualities (concurrent validity, convergent validity, reliability) were compared.

Results
The PCA resulted in eight components (Eigenvalue > 1.0). Item selection resulted in five candidate lists with 8, 9, 16 (2x) and 27 items. All lists showed good concurrent and convergent validity. Only one with 16 items combined good psychometric properties with the need for a shortened questionnaire combined with all clinically relevant items.

Conclusions
We recommend a novel 16-item GHB Withdrawal Symptoms Questionnaire (GWSQ) to monitor withdrawal during detoxification in patients with GUD. Implementation of this scale should be evaluated to determine its validity and usefulness in clinical practice.