Longitudinal measurement invariance of the international spinal cord injury quality of life basic data set (SCI-QoL-BDS) during spinal cord injury/disorder inpatient rehabilitation

Persons sustaining a spinal cord injury or disorder (SCI/D) are at increased risk to experience reduced quality of life (QoL) [1]. SCI/D is a severe impairment caused by physical trauma or disease [2] and the corresponding damage to the neurological tissue typically leads to an immediate, sometimes recovering, sometimes permanent complete or partial loss of body functions, mainly sensor motor and autonomic nerve function below the lesion level [3]. These functional impairments and the risk for developing various secondary health conditions such as chronic pain, fatigue, or muscle spasms [4] as well as resulting restrictions in daily activities and social participation [5] may additionally negatively affect QoL in persons with SCI/D. Thus, besides improving physical functioning and fostering community reintegration, increasing QoL is a key goal of first inpatient rehabilitation following SCI/D [6‐8].

Despite general agreement on the clinical relevance of QoL, there is no consensus on the definition of QoL within the SCI/D literature and as a result various measures have been used to assess it [6]. This limits comparability of findings from different studies and creates difficulties to draw any firm conclusions [9]. Therefore, an international group of experts developed the international spinal cord injury quality of life basic data set (SCI-QoL-BDS) [10] with the goal of standardizing the collection and reporting of QoL data in individuals with SCI/D. In addition, the measure aimed to be brief by assessing only a minimal amount of information to facilitate implementation in daily clinical practice. A brief QoL instrument is indeed especially useful in the inpatient rehabilitation phase to efficiently evaluate the evolution of patients’ QoL and the effectiveness of care. The SCI-QoL-BDS is based on a definition of QoL as individual’s subjective evaluation of how things are in their life [8]. This is operationalized with three items asking respondents to indicate to what degree they are satisfied with their life as a whole, their physical health, and their psychological health. Though the authors acknowledged that perceived QoL is a multifaceted construct covering also other domains, these were considered as the most relevant ones in the SCI/D context [10].

Since its development in 2012, the SCI-QoL-BDS has been employed by different research groups around the globe. Accordingly, the original English version was translated into other languages including Dutch [11], Brazilian Portuguese [12], German, French, Italian [13], and Thai [14]. Preliminary evidence from these studies revealed promising psychometric properties of the instrument as indicated by good internal consistency, convergent, and divergent validity among individuals with SCI/D during inpatient rehabilitation [15‐17] as well as among community-dwelling individuals [11, 14, 15, 18]. Two longitudinal studies further demonstrated acceptable to good test–retest reliability across a two-week interval [14, 18]. Nevertheless, more examination of the psychometrics properties of the SCI-QoL-BDS during first inpatient rehabilitation are still needed.

Aiming to extend previous research on the psychometric properties of the SCI-QoL-BDS, the objective of the present study was to test the SCI-QoL-BDS’s internal consistency and longitudinal measurement invariance from admission to discharge from SCI/D first inpatient rehabilitation. Testing longitudinal measurement invariance allows to determine whether the instrument assesses the same construct on the same metric at different points in time [19]. This is an important aspect for determining the instrument’s validity, consistency and a fundamental prerequisite to calculate change in the QoL construct and compare its structural relationships with other constructs over time [19, 20]. More specifically, if measurement invariance is not achieved, this indicates that respondents interpret the specific questions and/ or the underlying construct differently at different points in time [21]. As such, changes in scores over time do not necessarily represent quantitative differences in the construct itself. Instead, they may be the result of changes in the meaning of the construct over time (i.e., response shift) or they might be caused by different response styles over time [22, 23]. In sum, examining longitudinal measurement invariance of the SCI-QoL-BDS across first inpatient rehabilitation lays the ground for future research and clinical practice efforts aiming to evaluate the success of rehabilitation practices by measuring changes in the QoL of individuals with SCI/D from admission to discharge.

Descriptive statistics of the study participants are depicted in Table 1. To determine the representativeness of the study sample, participants were compared to non-participants with available data (Supplementary Table 1). No significant differences emerged with regard to physical functioning at T1, level and completeness of the injury. In contrast, participants included in the present study were significantly more likely male, younger, having a traumatic cause of the injury and a longer time to discharge from inpatient rehabilitation. However, the effect sizes in these comparisons were small (Cohen’s d between 0.07 and 0.27; Cramer’s V between 0.06 and 0.08) indicating only a slight selection bias in the study sample.

Descriptive statistics of the SCI-QoL-BDS item and total scores are displayed in Table 2. The distributions of the item scores and of the total score were slightly non-normal (Skewness between -0.6 and 0.2; Kurtosis between 2.2 and 2.7) at each of the three measurement time points. Overall, item and total scores tended to increase between inpatient rehabilitation admission and discharge. T-test comparing the SCI-QoL-BDS item and total scores between German-speaking participants and participants speaking other languages showed no statistical difference at any of the three time point.

The present study was the first to examine the internal consistency and different aspects of longitudinal measurement invariance of the SCI-QoL-BDS during first SCI/D inpatient rehabilitation. Supporting results from previous research [11, 14‐18], we found good internal consistency of the measure administered at one and three months after injury and at discharge from first inpatient rehabilitation. Using longitudinal factor analyses, we additionally demonstrated that the factorial structure and the factor loadings of the measure were invariant during first inpatient rehabilitation. This suggests that the three items of the SCI-QoL-BDS equally represent a latent QoL construct and that the meaning of this latent QoL factor seems to be stable across time [36, 37]. However, we also found some reasons for caution when using the instrument since only two out of the three items additionally proved to have invariant intercepts over time.

The item which was non-invariant at the intercept level is the respondents’ satisfaction with physical health. For this item, the intercept at one month post injury had to be freely estimated because it was substantially lower than at the two later measurement occasions. This means that the position on the latent QoL construct does not equally transfer to the observed level on this item over time. In other words, there is a systematic tendency for individuals with SCI/D to indicate lower satisfaction with physical health at one month post injury as compared to later measurement occasions and this tendency is not attributable to the concurrent position on the latent QoL variable [36, 37]. This might have resulted from a change in (most of) the respondents internal standards of measurement, so-called recalibration response shift [23, 38]. Individuals might have “recalibrated” their interpretation of the response options for the satisfaction with physical health item over the course of SCI/D inpatient rehabilitation. For example, at the beginning of inpatient rehabilitation, being totally satisfied with physical health might have required for individuals to have full physical functioning including the ability to walk and the physical health status before SCI/D onset might be used as reference framework to appraise satisfaction. At later stages of inpatient rehabilitation, individuals may have gained a better understanding of the primary physical consequences of the SCI/D as well as secondary health conditions and thus establish a new reference framework to evaluate their health. At this stage, positive responses to the item total satisfaction with physical health may reflect appreciation of absence of secondary health conditions and gains in physical well-being and functioning achieved during inpatient rehabilitation. Further longitudinal research, particularly qualitative studies examining the temporal differences in how individuals with SCI/D evaluate their satisfaction with physical health are needed to understand this potential response shift across inpatient rehabilitation.

As a side note, reevaluating one’s values and criteria for what constitutes good QoL can be an adaptive response to a life-changing event such as SCI/D. Studies of individuals who experienced cancer, loss, or other potentially traumatic events have documented changes in sense of self [39], appreciation of life [40], and life priorities [41]. This raises the possibility that the partial invariance observed in the present study actually might indicate good validity in the sense of reflecting a dynamic cognitive process rather than a static self-assessment, which would not capture the complexity of the psychological adaptation to potentially traumatic events such as the onset of an SCI/D.

The finding of partial intercept invariance has important methodological implications for future longitudinal studies. The response shift seems to take place in the very early phase of inpatient rehabilitation and intercept invariance can be observed in the two later time points. This indicates that the SCI-QoL-BDS might be used without longitudinal measurement issues in studies focusing on the later stages of inpatient rehabilitation spanning from the third month post SCI/D to rehabilitation discharge. Moreover, establishing partial intercept invariance is sufficient to allow for a meaningful interpretation of differences in the latent QoL factor means and their structural relations to other constructs across time [19, 21, 29]. Nevertheless, caution is needed when calculating and interpreting changes in the observed SCI-QoL-BDS total score (i.e., mean score) and in particular in the satisfaction with physical health during the early phase post SCI/D. Comparing observed means would require full intercept invariance [37]. Consequently, a latent variable framework such as latent change score models [42] may be best suited for examining the longitudinal course of QoL and its relationship with other constructs during the early rehabilitation phase [42].