To investigate if the Personality Disorder (PD) severity concept (Criterion A) of the ICD-11 and DSM-5 AMPD is applicable to children and adolescents, following the ICD-11 lifespan perspective of mental disorders, age-specific and informant-adapted assessment tools are needed. The LoPF-Q 6-18 PR (Levels of Personality Functioning Questionnaire Parent Rating) was developed to assess Impaired Personality Functioning (IPF) in children aged 6–18 in parent-reported form. It is based on the established self-report questionnaire LoPF-Q 12-18. Psychometric properties were investigated in a German-speaking clinical and school sample containing 599 subjects. The final 36-item version of LoPF-Q 6-18 PR showed good scale reliabilities with 0.96 for the total scale IPF and 0.90-0.87 for the domain scales Identity, Self-direction, Empathy, and Intimacy/Attachment and an acceptable model fit in a hierarchical CFA with CFI = 0.936, RMSEA = 0.078, and SRMR = 0.068. The total score discriminated significantly and with large effect sizes between the school population and (a) adolescent PD patients (d = 2.7 standard deviations) and (b) the younger patients (6–11-year-olds) with internalizing and externalizing disorders (d = 2.2 standard deviations). Informant agreement between parent and self-report was good at 0.47. Good construct validity can be assumed given sound covariation with related measures of psychopathology (CBCL 4-18, STiP-5.1, OPD-CA2-SQ PR) and maladaptive traits (PID5BF+ M CA IRF) in line with theory and matching the result patterns obtained in older samples in self-report. The results suggest that parent-reported assessments of IPF and maladaptive traits are equivalent to self-reported measures for Criterion A and B. Assessing IPF as early as age six might be a valuable step to foster early detection of PD, or maladaptive personality development, respectively individuals at risk.
