Measures of autistic traits are only useful — for pre-diagnostic screening, exploring individual differences, and gaining personal insight — if they efficiently and accurately assess autism as currently conceptualised whilst maintaining psychometric validity across different demographic groups. We recruited 1,322 autistic and 1,279 non-autistic adults who varied in autism status (non-autistic, diagnosed autistic, self-identifying autistic), gender (cis-male, cis-female, gender diverse), and age, to assess the psychometric properties of the Comprehensive Autistic Trait Inventory (CATI), a recently developed measure of autistic traits that examines six trait domains using 42 self-report statements. Factor fit for the six-subscales was appropriate, as was total-scale and subscale reliability. Importantly, measurement invariance was demonstrated based on both autism status and gender indicating that the CATI can be used to make direct comparisons between members of these groups. Autistic traits were highly similar between diagnosed and self-identifying autistic adults while gender-diverse participants showed more autistic traits than their cis counterparts. Sensitivity and specificity estimates supported the use of the CATI in identifying autistic from non-autistic participants, with discrimination thresholds calculated. Our analysis indicates that the CATI is a practical measure of autistic traits in non-autistic and autistic participants with utility for researchers, clinicians, and for affirming self-identity.