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Background Evidence-based mental health requires patient-relevant outcome data, but many indicators lack clinical meaning and fail to consider youth perceptions. The minimally important change (MIC) indicator designates change as meaningful to patients, yet is rarely reported in youth mental health trials.Objective This study aimed to establish MIC thresholds for two patient-reported outcome measures (PROMs), the Columbia Impairment Scale (CIS) and the Strengths and Difficulties Questionnaire (SDQ), using different estimation methods.Methods A sample of 247 youths (14–17 years) completed the CIS and SDQ at baseline and at 6 months in a youth mental health and substance use trial. At 6 months, youths also reported perceived change. Three anchor-based (mean change, receiver operating characteristic analysis, predictive modelling) and three distribution-based methods (0.5 SD, measurement error, smallest detectable change) were compared.Findings Different methods yielded varying MIC thresholds. Predictive modelling provided the most precise anchor-based MIC: −2.6 points (95% CI −3.6, –1.6) for the CIS and −1.7 points (95% CI −2.2, –1.2) for the SDQ, indicating that score improvements of 12% for the CIS and 8% for the SDQ may be perceived as ‘important’ by youths. However, correlations between change score and anchor were below 0.5 for both measures, indicating suboptimal anchor credibility. Stronger correlations between the anchor and T2 PROM scores compared with T1 scores suggest the presence of recall bias. All MIC estimates were smaller than the smallest detectable change.Conclusions Predictive modelling offers the most precise MIC, but limited anchor credibility suggests careful anchor calibration is necessary.Clinical implications Clinicians may consider the MIC CI as indicative of meaningful change when discussing treatment impact with patients.Trial registration number NCT02836080.