Confirming the factor structure of a generic quality of life instrument among pre-treatment substance use disorder patients

Background Quality of life (QoL) is a patient-reported outcome of increasing importance in the substance use disorder (SUD) treatment field, and impaired QoL may be an important impetus for treatment uptake. Instruments and methodologies abound, precluding comparison, as does a dearth of population norms. The QOL10 is a generic, overall QoL tool containing ten items and with simple scoring procedures. It is therefore a potential alternative to the gold standard WHOQOL-BREF. This study aimed to assess the two-factor structure of the QOL10 that has been suggested by a previous exploratory factor analysis. Methods Adults entering 21 participating inpatient or outpatient SUD treatment were recruited to join a national longitudinal cohort study. 531 completed the QOL10 at treatment entry and were included in the analysis. Structural equation modelling (SEM) was used to confirm the model fit of a two-factor structure, and the scaling qualities of the QOL10 were reported. Results According to the SEM analysis, the QOL10 was comprised of one latent variable measuring social QoL, and one measuring global QoL, and all ten items were retained. Goodness of fit tests included: root mean square of approximation = 0.063, 90% CI 0.050–0.076; normed-fit index = 0.919; and comparative fit index = 0.943. Conclusions The QOL10 should be considered when clinicians in the SUD treatment field need a short, valid instrument that measures both global QoL and social QoL, with minimum respondent and administrator burden. The social domain is of particular utility and may be used as a stand-alone instrument. Test-retest reliability should be established in future studies. Electronic supplementary material The online version of this article (10.1186/s12955-019-1152-7) contains supplementary material, which is available to authorized users.


Background
Quality of life (QoL) is an operationalization of the recovery model of substance use disorders (SUD) proposed by Substance Abuse and Mental Health Services Administration [1], and as such is an important treatment outcome measure [2]. As a necessarily patient-reported measure [3], QoL captures the lived impacts of SUD and treatment on a person's life, using information to which a physician or treatment provider is not privy [4]. The desire to improve QoL may be a more important motivation for treatment uptake than the reduction of substance use per se [5]. Given the wide-ranging social, medical, and legal consequences of SUDs, it is unsurprising that SUD patients have lower QoL than other chronic disease groups [6][7][8].
The International Society for Quality of Life Research suggests minimum standards for QoL instruments in addition to validity and reliability, such as a low burden to both respondents and administrators, i.e. using a minimum amount of items and simple scoring procedures [9]. Yet recent reviews of QoL measures among opioid users have highlighted that even existing instruments are being scored and presented differently [8,10]. The current gold standard among SUD patients is the World Health Organization's WHOQOL-BREF, a 26-item measure with four domains of physical health, psychological health, social relationships, and environment QoL [11]. Scoring instructions are not simple, however, and include syntax for the use of statistical software. A validation study reported that some of the negatively worded items may not have been understandable by substance users [12]. Another commonly used measure, the Lancashire Quality of Life Profile (LQoLP) [13], is psychometrically strong but imposes even more burden, with 133 items administered in a structured interview.
The generic, multidimensional QOL10 [14] may be a less burdensome and more acceptable alternative for the SUD population. The QOL10 has fewer items (ten) than the WHOQOL-BREF, no negatively worded items (compared to three), and the scoring procedure is simple. A previous analysis on a small sample demonstrated convergent validity to the WHOQOL-BREF and suggested that a two-domain structure of "social QoL" and "global QoL" was a better fit than the QOL10's original hypothesized three-domain structure [15]. This previous analysis also reported that the QOL10's social QoL domain had higher internal reliability than the WHOQOL-BREF's social relationships domain. Exploration of the QOL10 with a larger sample size as well as more sophisticated analytic methods are needed.
This analysis confirms a two-domain structure of the QOL10 instrument, and reports on item responses, mean values, and scaling qualities. Data are drawn from 531 patients entering heterogeneous SUD treatment programs across Norway.

Design and setting
Data from this analysis is drawn from the longitudinal Norwegian Cohort Study of Patients in Opioid Maintenance Treatment and Other Drug Treatment (NorComt) Study. Participants were recruited to NorComt upon entry to any of the 21 participating treatment facilities, with no exclusion criteria, between 2012 and 2015. After collecting written informed consent, treatment providers who had been trained by the research team administered a 100 -item questionnaire to entering patients. The questionnaire included the QOL10, described below, and other validated measures such as the Europe-ASI [16] and Hopkins Symptoms Checklist 25 [17]. Sixty to ninety minutes on average were spent filling out the questionnaire.

Participants
Participant characteristics of the entire NorComt study population at treatment and study entry have been reported on previously [18,19]. Briefly, 531 of 548 provided valid QOL10 scores (described in "Quality of life instrument" below) and were included in this analysis.

Quality of life instrument
The QOL10 includes ten items asking for participants' current evaluation of various aspects of their life, such as "how is your working ability at the moment?" and "how are your relationships with your friends at the moment?" All items are answered on a 1-5 Likert-type scale from "very poor" to "very good", with a neutral option. In a previous study of the QOL10 using a subsample of the first NorComt participants followed up with after one year, during 2013-2015, an exploratory factor analysis suggested that a two-factor structure of a social QoL domain and a global QoL domain was a good fit [15]. The social QoL domain contained five items (item 4: friends, 5: partner, 6: ability to love, 7: sexual functioning, and 8: social functioning), while the global QoL domain contained the remaining five items (item 1: physical health, 2: mental health, 3: feel about yourself, 9: work, and 10: overall QoL). Both social and global QoL domain scores were converted to 0-100 scales to be comparable with the WHOQOL tools [11], and both domains had acceptable internal consistency (α = 0.814 for social QoL, and α = 0.771 for global QoL) The WHOQOL Group suggests an upper limit of 20% missing data for the calculation of domain scores, therefore all five items in the global QoL domain were required. In the social QoL domain, only the partner item was allowed missing, to avoid excluding single participants, and the social QoL domain calculated based on the remaining four items. Seventeen participants were deleted listwise due to lacking one or both domain scores, leaving 531 participants in the analysis.

Analysis
Structural equation modelling (SEM) is a sophisticated multivariate procedure that tests a priori relationships between observed and latent variables in a structural theory [20]. Utilizing a confirmatory factor analysis approach and maximum likelihood estimation procedure, SEM was used to assess a two-factor structure suggested by the earlier exploratory factor analysis [15], with the friends, partner, ability to love, sexual functioning, and social functioning observed variables loaded onto the latent variable social QoL, and the physical health, mental health, feel about yourself, work, and overall QoL observed variables loaded onto the latent variable global QoL. Covariance paths were entered between the two latent variables. Model fit was assessed by four indices, as suggested by Kline [21]: the model chi-square, a traditional measure of overall fit that is sensitive to sample size and assumes multivariate normality (p > 0.05 represents a good fit); two alternative indices to the chi-square, the normed-fit index, indicating the improvement in fit of the model of interest relative to the null model, and comparative fit index, a revised form of the normed-fit index that is less sensitive to sample size (values ≥0.90 represent a good fit); and the root mean square error of approximation, a parsimony-adjusted index in which lower values are preferable (values ≤0.05 represent a good fit, and confidence limit upper values should be ≤0.08), Hoelter's statistic assessed the adequacy of sample size (≥200) [22]. All statistics were performed on SPSS AMOS v25. Table 1 displays the scaling qualities of the QOL10. On a scale of 0-100, the mean social QoL domain was 53.5 (SD 20.0), and the mean global QoL domain was 32.9 (SD 19.1). Both domains were non-normally distributed due to positive skews, but examination of the normal Q-Q plots determined non-normality to be modest. Additional   Table S1 displays the response distribution to each of the ten items, and Additional file 2: Table S2

Discussion
Using data from a large national treatment study of SUD in Norway, this analysis aimed to explore the factor structure of the generic QOL10 instrument administered to 531 patients entering a variety of substance use disorder treatments. The QOL10 was confirmed to measure social QoL and global QoL with five items in each domain and a stable factor structure [15]. The overall QoL of this sample in a previous study was found to be severely impaired [18], as has been extensively reported among this patient group [7,8,23]. Our sample's social QoL (53) was slightly higher than their global QoL (33), a pattern also found in a large sample of opioid use disorder patients beginning treatment in Germany [24]. In the German study, a different instrument with the same scoring scale was used, and three social domain scores (psychosocial, partner, and family domains) ranged from 42 to 57, while the general QoL domain score was 38. These results should not be interpreted as this sample or others with substance use disorders having acceptable social QoL, rather, that their global QoLaccounting for health, working ability, and an overall QoL evaluationis extremely impaired.

Conclusions
The QOL10 is a valid instrument that could be considered an even shorter alternative to the gold standard WHOQOL-BREF, particularly when the social domain of QoL is of interest. The QOL10's social domain had higher internal reliability than the WHOQOL-BREF's corresponding domain [15], and by including two additional items, may collect more relevant information than the WHOQOL-BREF without significant extra burden. The social QoL domain of the QOL10 may also be extracted and used independently, a particularly helpful feature of this instrument given that the stigma associated with SUDs, including treatment-seeking, can reduce this domain [23,25], while social support may protect it [26][27][28]. Attention to social QoL in a clinical setting could expose areas for intervention, such as social networks, a lack of abstinence-specific social support, or anti-stigma training among health care professionals. More research in general is needed on social QoL, and psychometrically strong instruments are the first step.

Additional files
Additional file 1: