Psychometric assessment of the Swedish version of the injustice experience questionnaire among patients with chronic pain

Objectives: The use of the Injustice Experience Questionnaire (IEQ) in psychological assessment of individuals with chronic pain is supported by research. The psychometric properties of the Swedish version, the IEQ-S, has not yet been evaluated. Hence, the aim was to investigate structural validity, and concurrent criterion validity of the IEQ-S against the Work Ability Index (WAI), the Pain Catastrophizing Scale (PCS-SW), the Patient Health Questionnaire 9-item depression module (PHQ-9), and the Generalized anxiety disorder 7-item scale (GAD-7). Internal consistency and test-retest reliability were also studied. Methods: Sixty-five participants, referred to a University hospital, with a pain duration over three months were consecutively sampled. They completed the IEQ-S at admission and again within six weeks. A confirmatory factor analysis was performed for the study of structural validity. Concurrent criterion validity was evaluated using Spearman's correlation coefficient. Internal consistency reliability for the full IEQ-S was calculated using the Cronbach's alpha. Test-retest reliability was calculated using an Intraclass Correlation Coefficient (ICC). Results: The median total score (0-48, where high scores indicate high levels of injustice) at admission (test 1) was 27.0 (n=64), 25th percentile=15.3, 75th percentile=37.8, range=3-48 points. A one-factor model was supported with item-loadings between 0.67-0.92. Spearman's correlation coefficient between the IEQ-S and the WAI (n=56) was r(s)=-0.46; the PCS-SW (n=63) was r(s)=0.68, the PHQ-9 (n=64) was r(s)=0.50 and the GAD-7 (n=64) was r(s)=0.57, p<0.01. Cronbach's alpha was 0.94 (n=64). The ICC was 0.80 (n=55), with a 95% confidence interval, ranging between 0.69-0.88. Conclusions: Our study supported structural validity and concurrent criterion validity of the IEQ-S against other measures of psychological constructs and work ability. It also supported the internal consistency reliability of the IEQ-S and the test-retest reliability with a retest interval up to six weeks, was good. These findings support the use of the IEQ-S as an adjunct tool to assess appraisals of injustice in patients with chronic pain who are referred to tertiary care in Sweden. The added value might be identification of those who are at risk for slow or no improvement in their pain condition over time, and sick-leave, but this has to be confirmed in future studies.