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

Setting and sample

This study was part of a feasibility study for a large cohort within the U-PAIN research program [14], which is an interdisciplinary and translational pain research program exploring methods for pain diagnostics, treatment and rehabilitation. The aim of the cohort is to identify patients who benefit from long-term opioid therapy or are at risk for problematic opioid use. A multidisciplinary research group will follow over 1,000 patients over the course of five years with annual follow-ups to build a knowledge base regarding risks and benefits with long-term opioid treatment. The study was approved by the regional ethics board in Uppsala (EPN Uppsala D-No 2016-376).

The sample was consecutive and set to 65 participants for this feasibility study. The sample included patients with chronic pain, referred to tertiary care at a pain clinic in a Swedish university hospital during 2019. Inclusion criteria were: an age of 18 years or older, first visit of their current referral to the pain clinic, and a pain duration of more than three months at the time of referral. Patients who were given acute care related to active cancer treatment and patients in palliative care were excluded, as were patients who had cognitive impairment or were illiterate in the Swedish language. According to the government agency Swedish Statistics, the Swedish working population constitutes of inhabitants between the ages of 15–74 [15]. Since we wanted to explore work ability with regard to concurrent criterion validity, participants over the age of 74 were hence excluded from this analysis. Participants who had failed to answer all, or the majority of, questions in any questionnaire, were excluded from the particular analysis where this questionnaire was needed. When a participant had failed to answer a minor part of a questionnaire, and this response did not affect the total score or index, a deductive imputation [16] was made and the participant was included.

Measures

Procedures

The participants received a written invitation letter to the pain clinic in which they were asked to fill in the questionnaires for admission (test 1) to prepare for their clinical visit. They were instructed to do so electronically, using the Swedish Healthcare Guide online. In the invitation letter, they received written information about the study. Research staff called the participants before their clinical visit to offer help with the above described procedure and requested permission to give more information about the cohort and feasibility study during the clinical visit. Patients accepting to participate filled in a written consent form. The first 65 patients who met the inclusion criteria and accepted to be part of the feasibility study were included. This sampling was made between January and June 2019. The participants were then asked to complete the IEQ-S once more for a retest (test 2) at their clinical visit. These data had to be done within six weeks after test 1 to be included in the analysis. The retest (test 2) was mainly done through a web-based platform for distribution of electronic questionnaires.

Data analysis

A confirmatory analysis was conducted to investigate the strength of the 12 items loadings. Data from the first test were analyzed. The packages psych and nFactors in R 4.0.2 was used for the factor analysis.

The concurrent criterion coefficients between the IEQ-S and WAI, PCS-SW, PHQ-9 as well GAD-7 were all calculated using Spearman’s correlation coefficient. The correlation coefficient was interpreted as zero (0.0), weak (0.1–0.3), moderate (0.4–0.6), strong (0.7–0.9), and perfect (1.0) correlation [34]. The internal consistency reliability was established by calculating Cronbach’s alpha, including the Corrected Item–Total Correlation and Cronbach’s Alpha if Item is deleted, for the IEQ-S total scores at admission (test 1). A Cronbach’s alpha around 0.8 was desirable [35]; however, a value over 0.9 was considered as suggesting that the different items might be too similar [36]. Having the Corrected Item–Total Correlation (correlations between each item and the total score of a measure) be larger than 0.30 was desirable. The Cronbach’s Alpha if Item is Deleted for each individual item should not be higher than the overall alpha, since this would have meant that the deletion of that item would result in higher reliability [35]. An Intraclass Correlation Coefficient, 2-way mixed effects model, with absolute agreement definition, was used as the test-retest reliability coefficient. The ICC value was interpreted as poor (<0.5), moderate (0.50–0.75), good (0.75–0.9), and excellent (>0.9) reliability [37]. The analyses were made using the Statistical Package for the Social Sciences (SPSS version 26).

Missing data

When scrutinizing data, it was discovered that one of the included participants had a pain duration of less than three months and was thus excluded from all analyses. Eight participants were excluded from the concurrent criterion validity analysis between the IEQ-S and the WAI, whereof five participants had not answered the WAI, two participants had not answered a majority of the WAI questions and one participant was over age 74. One participant had failed to answer one question of the WAI and a deductive imputation could be made. Another participant had failed to answer the majority of the questions of the PCS-SW, hence was excluded from the concurrent criterion validity analysis between the IEQ-S and the PCS-SW. Finally, three participants had not answered the IEQ-S retest (test 2) and six participants had not answered within the pre-determined interval (six weeks or less), which excluded them from the test-retest reliability analysis. Figure 1 displays a flowchart of the included and the excluded data.

Figure 1:

Flowchart of included and excluded data.

Sample characteristics

Of the participants, 42.2% (n=27) were men and 56.3% (n=36) were women; one participant was unsure about gender identity. Their age ranged from 19 to 85 years old, with a mean of 50.3 (n=64) and a standard deviation (SD) of 14.5. The median total score for the IEQ-S measured at admission (test 1) was 27.0 (n=64), 25th percentile=15.3, 75th percentile=37.8, with a range between 3–48 points (IEQ total score=0–48, where higher scores indicate higher levels of injustice). The mean was 26.2 with a standard deviation of 13.3. 30 out of 64 participants (46.9%) had a total score of ≥30, representing a clinically relevant level of perceived injustice. Forty-one out of 64 participants (64%) had a total score ≥19, which may represent a barrier to return to work. 30 participants (46.9%) had a total score of ≥30, representing a clinically relevant level of perceived injustice. Forty-one participants (64%) had a total score ≥19, which may represent a barrier to return to work. Demographic variables and clinical characteristics are summarized in Table 1.

Structural validity

In a one-factor confirmatory factor analysis all items had high loadings, see Table 2, which indicates that all items load on the same factor. Testing for the number of factors it was confirmed that a one-factor model was a good fit. The scree test showed the second factor had an eigenvalue just below 1. The proportion of variance explained was 0.61.

For exploratory purpose the two-factor model was also fitted, which resulted in great overlap in item content between the two factors.

Concurrent criterion validity

The correlation coefficient between the IEQ-S and WAI (n=56) was r_S=−0.46, p<0.01, meaning that there was a negative, significant correlation between perceived injustice and work ability. The correlation coefficient between the IEQ-S and PCS-SW (n=63) was r_S=0.68, p<0.01, meaning that there was a positive significant correlation between perceived injustice and pain catastrophizing. The correlation coefficient between the IEQ-S and PHQ-9 (n=64) was r_S=0.50, p<0.01, meaning that there was a positive significant correlation between perceived injustice and depression. Lastly, the correlation coefficient between the IEQ-S and GAD-7 (n=64) was r_S=0.57, p<0.01, meaning that there was a positive significant correlation between perceived injustice and generalized anxiety. In summary, high levels of perceived injustice were associated with low work ability and high levels of pain catastrophizing, depression, as well as generalized anxiety.

Internal consistency reliability

The Cronbach’s alpha for the overall IEQ-S (n=64) was 0.941. The Corrected Item-Total Correlation ranged between 0.619–0.894, and the Cronbach’s Alpha if Item Deleted varied between 0.929–0.940 for the 12 items, as shown in Table 3.

Test-retest reliability

The ICC, single measure, 2-way mixed effects model, with absolute agreement definition, was 0.80 (n=55) with a 95% confidence interval, ranging between 0.69–0.88. Table 4 shows the central tendency and variability of the total scores for IEQ-S on the two different occasions. The number of days between the test at admission (test 1) and retest (test 2) varied between 3–42 days, with a median of 22 days.

Methodological considerations

The results have to be interpreted with some limitations in mind. The sample was recruited consecutively for a specific sample size, resulting in a sampling period of approximately six months. If the sample had been determined by a specific, longer, time period, the risk of potential bias could have been smaller, as it would have reduced biases due to seasonal or other time-related fluctuations [38]. There is a lack of scientifically sound recommendations for sample size determination for psychometric studies [39, 40]. However, the more homogeneous the sample, the smaller the sample size required [38]. The sample in this study can be considered heterogeneous with respect to many of the demographic variables presented, which may motivate that a larger sample size would have been desirable. This is particularly relevant for the confirmatory factor analysis, which should be seen as preliminary. At the same time, all participants had chronic pain and were referred to Swedish tertiary care, which reflect an aspect of homogeneity, and could warrant the sample size chosen. Future research should study larger samples to get more insight into how injustice is perceived among patients with chronic pain.

A limitation with our concurrent criterion validity analysis between the IEQ-S and the WAI was that some of the participants did not have a current employment, making the assessment of work ability more difficult. About 8% of the participants stated that they were job seekers or in a labor market policy measure. Almost a third of the participants were on long-term sick leave. Whether or not they had a current employment was not specified in our background data. An additional instruction was added to the WAI to measure work ability for individuals with no current employment in this study, asking them to answer the questions based on the profession and the main work tasks of a previous employment, which might have introduced recall bias.

The number of days between the test at admission (test 1) and retest (test 2) in this study was not consistent, which might have affected the IEQ-S stability negatively. In this study, the test-retest interval was not decided in consideration to the specific questionnaire or variable analyzed since it was part of a large cohort study. The interval ranged from three to 42 days and depended on when the participants had their clinical visit scheduled, which was when they completed the retest (test 2). There is risk that participants who had short intervals remembered their initial answer, which might have affected their second answer. On the other hand, the risk that their perceptions of injustice had changed decreased [41]. With longer intervals, there is a risk that the test-retest reliability has naturally declined, which it tends to do even with more stable traits [38].

The IEQ was originally developed to assess individuals who have suffered an injury, asking the respondents to reflect on how their injury has affected them. A new, modified version of the IEQ, the IEQ-chr, has been created to target individuals who have not necessarily sustained their pain from a specific injury. In the IEQ-chr, the respondents are asked to reflect on how their health condition has affected their lives [4]. This new version might have been better suited for the sample of our study; nonetheless, the item content as well as the response format of the IEQ-chr are identical to that of the original IEQ.

Conclusions

This study has contributed with new information about the psychometric properties of the IEQ-S, for patients with chronic pain referred to tertiary care in Sweden. Our study supported structural validity of the IEQ-S based on a one-factor model. The concurrent criterion validity of the IEQ-S in relation to measures of work ability, pain catastrophizing, depression, and generalized anxiety was supported, as was the internal consistency reliability of the IEQ-S. The test-retest reliability of the IEQ-S, with a retest interval up to six weeks, was good.

Much is yet to be understood about the relationship between perceived injustice and other biopsychosocial factors in the context of chronic pain. Larger samples should be studied to get more insight into how injustice is perceived among patients with chronic pain. Future research should study the IEQ-S total score in association with other variables, such as the concurrent criterion validity between injustice and pain, and how perceptions of injustice vary depending on different onsets of pain. The predictive validity of the IEQ-S in relation to work ability should also be studied. A final remark is the need for further validation of Swedish versions of established questionnaires commonly used in different pain populations.

Clinical implications

The complexity of chronic pain requires a consideration of biopsychosocial factors when assessing the severity of the condition. Our study supports 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 the pain condition over time and sick-leave, but this has to be confirmed in future studies. There would also be possibilities to specifically target patients with high levels of injustice in psychological treatment and pain rehabilitation.