Background Orthorexia nervosa (ON) has attracted increasing attention in recent years, and the Orthorexia Nervosa Inventory (ONI) is one of the most widely used instruments for assessing ON. As research on ON expands across different cultural contexts, multiple countries have conducted translation and cultural adaptation studies of the ONI. However, the methodological quality of these translation procedures and the psychometric properties of the adapted versions have not yet been systematically synthesized and compared. Guided by the COSMIN framework, this systematic review aimed to evaluate the measurement quality of cross-culturally adapted ONI versions and to provide evidence for the standardized use of the ONI in clinical and research settings. Methods This systematic review followed the COSMIN and PRISMA recommendations and was prospectively registered in PROSPERO (CRD420251218905). Using “Orthorexia Nervosa Inventory” and related terms, we systematically searched PubMed, Scopus, the Web of Science, Embase, and the China National Knowledge Infrastructure (CNKI) up to November 2025. We included peer-reviewed original studies published in English or Chinese that reported the translation and/or cultural adaptation of the ONI and its psychometric properties. Furthermore, two reviewers independently performed study selection and data extraction. Translation and adaptation methods were appraised using “Guidelines for the Process of Cross-Cultural Adaptation of Self-Report Measures” from Beaton et al. Measurement properties were evaluated using the quality criteria of Terwee et al. and the COSMIN Risk of Bias checklist across domains including structural validity, internal consistency, reliability, measurement error, construct validity, and responsiveness. A narrative synthesis was conducted. Results The search yielded 280 records, of which five studies met the inclusion criteria, covering four countries and five language versions of the ONI (Chinese, Italian, Hungarian, and two Turkish versions). There was substantial heterogeneity in translation and cultural adaptation procedures. Only one Turkish version fully reported all five steps recommended by Beaton’s guidelines. Approximately 80% of the studies fulfilled the requirements for forward–backward translation and synthesis, whereas only approximately 40% reported an expert committee review and 40% reported pretesting. According to COSMIN ratings, approximately 60% of the studies were judged “very good” for structural validity, with the majority of versions supported by exploratory factor analysis (EFA) and/or confirmatory factor analysis (CFA) that broadly confirmed the intended factor structure. All five studies reported acceptable internal consistency (Cronbach’s α or ω for the total scale and subscales was within the recommended ranges). However, only the Chinese version provided data on test–retest reliability and measurement error/minimal detectable change (MDC), and measurement error was primarily estimated via the standard error of measurement (SEM) derived from Cronbach’s α, without interpretation against a minimal important change. Evidence for construct validity was limited: Only a subset of studies tested predefined hypotheses using correlations with related measures or known-groups comparisons, and these studies were mostly rated as “adequate” or “doubtful.” None of the included studies conducted multi-group factor analysis or differential item functioning (DIF) to assess cross-cultural validity/measurement invariance, and no study reported responsiveness or ROC-based cut-off determination. Conclusion Existing cross-cultural versions of the ONI demonstrate generally good internal consistency and preliminary support for structural validity. Nonetheless, important gaps remain in the rigor of translation procedures, test–retest reliability, measurement error, and construct validity, with a striking absence of evidence on cross-cultural validity and responsiveness. Future research should rigorously adhere to established cross-cultural adaptation guidelines and COSMIN methodological standards, incorporate multi-group CFA to examine measurement invariance, and comprehensively assess test–retest reliability, measurement error, and responsiveness. In addition, clinically and functionally meaningful cut-off scores should be established to enhance the applicability and comparability of the ONI across diverse cultural contexts. Systematic review registration https://www.crd.york.ac.uk/PROSPERO/view/CRD420251218905 , Identifier: CRD420251218905.
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