¹Seoul Metropolitan Government-Seoul National University Boramae Medical Center, Department of Psychiatry, Seoul, Korea, Republic of, ²Seoul National University College of Medicine, Department of Psychiatry, Seoul, Korea, Republic of, ³Nowon Eulji Medical Center, Department of Psychiatry, Seoul, Korea, Republic of, ⁴Uijeongbu Eulji Medical Center, Department of Psychiatry, Uijeongbu, Korea, Republic of

Background: Clozapine is the most effective antipsychotic for treatment-resistant schizophrenia, yet its clinical use is frequently limited by a distinct profile of adverse effects, including hypersalivation, constipation, urinary disturbances, and cardiovascular symptoms. These side effects contribute substantially to poor adherence and premature discontinuation. Conventional clinician-rated instruments, such as the UKU Side Effect Rating Scale, are comprehensive but time-consuming and may insufficiently capture patients’ subjective burden. To address these limitations, the Glasgow Antipsychotic Side-effects Scale for Clozapine (GASS-C) was developed as a brief, patient-reported outcome measure specifically targeting clozapine-related adverse effects. However, a culturally and linguistically validated Korean version has not previously been available.

Aims & Objectives: The aim of this study was to translate and standardize the Korean version of the GASS-C and to evaluate its psychometric properties. The specific objectives were to assess (1) internal consistency, (2) test–retest reliability, (3) criterion validity against established side-effect scales, and (4) clinical correlates with quality of life, functioning, and clozapine dose and plasma levels.

Method: This multicenter validation study was conducted at two general hospitals in Korea. Participants were adults (≥18 years) diagnosed with schizophrenia spectrum disorders according to DSM-5 criteria and currently receiving clozapine treatment. A total of 103 patients were enrolled. The GASS-C was translated using forward–backward translation. Participants completed the Korean GASS-C and the Liverpool University Neuroleptic Side Effect Rating Scale (LUNSERS), while clinicians administered the UKU Side Effect Rating Scale. Symptom severity and functioning were assessed using standardized clinical scales, including quality of life and personal and social performance measures. Clozapine dose and plasma concentrations (clozapine and norclozapine), when available, were collected. Test–retest reliability was evaluated in a subsample of 30 patients reassessed after 4–8 weeks on a stable dose. Statistical analyses included Cronbach’s alpha, correlation coefficients, and criterion validity analyses.

Results: The Korean version of the GASS-C demonstrated excellent internal consistency, with Cronbach’s alpha values ranging from 0.86 to 0.88. Test–retest reliability was good (r = 0.747, p < 0.001). Criterion validity was supported by strong correlations with LUNSERS total scores (γ = 0.790, p < 0.001) and moderate to strong correlations with UKU total scores (γ = 0.641, p < 0.001). Higher self-reported side-effect burden was significantly associated with poorer quality of life and reduced functional outcomes. GASS-C scores were not significantly correlated with clozapine dose or plasma levels, whereas clinician-rated UKU scores, particularly autonomic side effects, showed significant associations with dosage.

Discussion & Conclusions: These findings indicate that the Korean GASS-C is a reliable and valid patient-reported measure for assessing clozapine-related adverse effects. This study presents the first standardized Korean version of the GASS-C, demonstrating robust psychometric properties and clear clinical relevance. The scale offers a rapid, patient-centered tool for systematic monitoring of clozapine side effects and has the potential to improve treatment adherence and safety. Its application in future multicenter studies and integration into hospital information systems may further enhance clozapine management in real-world clinical settings.