Discussion

The findings of this COSMIN-based review indicate that S-QoL41, S-QoL18, and S-QoL-MCAT constitute a coherent family of patient-reported outcome measures with psychometric performance that is sufficiently robust to justify their use in both clinical research and routine care for people with schizophrenia. For structural validity, internal consistency, test–retest reliability, construct validity, and responsiveness, multiple studies of adequate or very good methodological quality converge on positive findings1,2,6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,21,22; accordingly, the overall certainty of evidence for these properties was graded as moderate. However, evidence for cross-cultural validity and measurement error remains limited, with few studies undertaking formal invariance testing or measurement error analysis, leading to low to very low certainty. Beyond the COSMIN-defined psychometric properties, studies employing neuroimaging reported associations between S-QoL scores and relevant brain regions, providing preliminary contextual evidence consistent with the biological relevance of the instrument's multidimensional structure.23,24

The findings confirm the stability of an eight-dimensional, patient-derived QoL construct across S-QoL41, S-QoL18, and S-QoL-MCAT, with clinically plausible associations to symptoms, functioning, and other QoL measures. Item reduction and computerized adaptive administration have preserved construct coverage and responsiveness. However, evidence for full cross-cultural invariance remains insufficient, with current support largely limited to structural consistency across language versions. These results reinforce a patient-centred conceptualisation of QoL in schizophrenia, rooted in Calman's expectancy-discrepancy model and developed through non-directive patient interviews that prioritised domains such as autonomy, resilience, and sentimental life beyond symptom-focused outcomes.1,27 Psychometrically, the consistency of the eight-factor structure across multiple settings suggests a broadly stable multidimensional construct,11,12,14,21 although formal equivalence across cultures cannot yet be confirmed. Studies in cognitively impaired and severely ill populations further support the robustness of S-QoL structure and item relevance across clinical severity strata.8,9

The S-QoL-MCAT demonstrates the application of item response theory and computerized adaptive testing to reduce respondent burden while preserving measurement precision, particularly relevant in cognitively impaired or severely symptomatic populations.6 The three instruments differ in their balance between measurement comprehensiveness, feasibility, and mode of administration. S-QoL18 appears to offer a favourable balance between measurement performance and feasibility, S-QoL41 provides a more comprehensive assessment when greater detail is required, and S-QoL-MCAT offers an efficient adaptive format with high measurement precision but more limited cumulative evidence. Together, these results support the use of S-QoL instruments as core measures in recovery-oriented care and research.

Functional and structural neuroimaging studies demonstrate that key dimensions, including psychological well-being, autonomy, and social relationships, are associated with activity and microstructural integrity in brain regions implicated in emotion, decision-making, and social cognition.23,24 These findings provide preliminary contextual evidence regarding the biological correlates of S-QoL domains. In addition, associations with functional outcomes further support clinical validity.28 Together, patient-derived item generation and exploratory neurobiological findings provide complementary contextual information within COSMIN's conceptual framework.29 Clustering analyses identifying reproducible QoL strata (high, moderate, low) further suggest meaningful latent structure aligned with clinical severity and functioning.29 Collectively, these data support S-QoL instruments as patient-centred measures of QoL in schizophrenia.

Beyond schizophrenia, the validity and reliability of S-QoL18 have been confirmed in other psychiatric populations. Boyer et al. (2022)30 demonstrated that S-QoL18 is a psychometrically sound tool for assessing QoL in patients with bipolar and depressive disorders, with the eight-factor structure confirmed via confirmatory factor analysis and significant correlations with symptomatology and functioning. Similarly, Girard et al. (2016)25 showed that S-QoL18 can be applied to homeless individuals with bipolar disorder or schizophrenia, maintaining adequate internal consistency, external validity, and sensitivity to change. These findings suggest potential cross-diagnostic applicability of S-QoL18, although further validation across heterogeneous settings is required. Equity and inclusion are notable features of the S-QoL programme, given that development and validation studies included individuals with severe illness, cognitive impairment, and homelessness.7, 8, 9,14 However, psychometric evidence remains geographically concentrated, with limited data from African, South Asian, and other under-represented regions, limiting global generalisability. Evidence from cognitively impaired and socially marginalised populations supports feasibility and structural robustness across vulnerable groups. This participatory approach aligns with rights-based and recovery-oriented frameworks and enhances the acceptability and relevance of the instruments to service users.1

Several limitations should be considered. Heterogeneity in study designs, sample sizes, and analytic approaches necessitated a primarily descriptive synthesis of psychometric indices, limiting precision and precluding formal exploration of statistical heterogeneity. Evidence on cross-cultural validity and measurement error was limited and frequently based on incomplete invariance testing, resulting in low to very low certainty ratings due to insufficient formal measurement invariance testing across cultural groups, limiting confidence in cross-cultural score equivalence. Consequently, cross-national and cross-group comparisons of S-QoL scores should be interpreted with caution, particularly when used for cross-country comparisons or clinical interpretation of absolute scores, as unresolved measurement invariance may bias inferences across populations. This limitation further limits the generalisability of the recommendations across different cultural and linguistic contexts. In addition, inter-rater reliability was not reported in any of the included studies; therefore, the evidence base is limited to test–retest reliability only. Criterion validity for schizophrenia-specific QoL was seldom evaluated, and the absence of a widely accepted gold standard for this construct constrained the interpretability of criterion-based assessments. Finally, data on real-world implementation and performance of the S-QoL-MCAT in routine care remain sparse, with evidence derived from one cross-sectional based on simulations rather than large-scale clinical deployment.

Future research should prioritise robust cross-cultural validation across diverse countries, languages, and socio-demographic groups, alongside longitudinal studies to clarify responsiveness and minimal important differences across treatment modalities and illness stages. Measurement error in real-world clinical contexts requires further investigation, and implementation research should explore integration of S-QoL18 and S-QoL-MCAT into routine care and digital platforms, assessing feasibility, acceptability, and impact on decision-making.31 In the absence of established minimal important differences or smallest detectable change, score changes should be interpreted cautiously at the individual level and in conjunction with clinical assessment and longitudinal trends, rather than relying on absolute change thresholds. Co-design with service users, carers, and clinicians will be essential to ensure that future refinements address evolving priorities such as stigma, social determinants of health, and digital connectivity.

This COSMIN-based systematic review shows that the S-QoL measurement system demonstrates overall acceptable evidence supporting its use as a patient-centred approach to assessing health-related quality of life in schizophrenia. Across studies, evidence for key measurement properties including structural validity, reliability, construct validity, and responsiveness was generally supportive, although methodological quality and completeness of evidence varied across domains.

Evidence was derived from diverse clinical populations, including individuals with cognitive impairment, severe illness, and homelessness, as well as from multiple language versions; however, geographic coverage remains limited, restricting global generalisability. Persistent uncertainties regarding cross-cultural validity and measurement error underscore the need for further high-quality studies, particularly in under-represented regions and populations. Strengthening the evidence base for S-QoL instruments and integrating rigorously evaluated patient-reported outcome measures into mental health systems may support a shift towards recovery-oriented care focused on autonomy, functioning, and subjective well-being.