BACKGROUND: Standardised psychometric measures are used in mental health care and research settings to identify risk, assist diagnosis, and assess symptom severity. Standardised scoring of these measures involves transforming respondents' raw scores using binary sex norms. However, scoring manuals offer no guidance as to appropriate scoring methods for trans and non-binary respondents. The Child and Adolescent Health Service - Gender Diversity Service (GDS) implemented a practice to score psychometric measures twice, according to both birth-registered sex (BRS) and the other sex (OS) norms (i.e., "double-scoring"). As it was unclear which choice of norm was the most clinically useful for the identification of risk, this study investigated whether there was any clinically meaningful difference in levels of psychopathology when scoring measures by OS versus BRS norms. METHODS: We analysed clinical-range classification of scores (clinically significant versus sub-clinical) on four sex-normed psychometric assessment measures (Child Behavior Check List, Youth Self Report, Social Responsiveness Scale 2, and Beck Youth Inventory) completed by GENder identiTy Longitudinal Experience (GENTLE) participants, as part of standard initial assessment at the GDS (N = 233). RESULTS: Sub-domains within all four measures resulted in inconsistent classifications of between 0 and 15% of participants into clinical versus sub-clinical ranges when scored using OS versus BRS norm. CONCLUSION: Scoring sex-normed standardised psychometric measures by BRS or OS results in different scores, in varying domains, and not in a uniform way. For clinical use, we suggest it is preferable to err on the side of sensitivity. For research purposes, we recommend standardised use of birth-registered sex norms for current measures. We suggest developing universal non-gendered psychometric measures which rate psychological symptoms according to distress and functional impairment, resulting in a more inclusive, equitable framework for evaluating mental health in gender diverse populations.