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Aim Increasing literature has shown the usefulness of a dimensional approach to autism. The present study aimed to determine the psychometric properties of the Adult Autism Subthreshold Spectrum (AdAS Spectrum), a new questionnaire specifically tailored to assess subthreshold forms of autism spectrum disorder (ASD) in adulthood. Methods 102 adults endorsing at least one DSM-5 symptom criterion for ASD (ASDc), 143 adults diagnosed with a feeding and eating disorder (FED), and 160 subjects with no mental disorders (CTL), were recruited from 7 Italian University Departments of Psychiatry and administered the following: SCID-5, Autism-Spectrum Quotient (AQ), Ritvo Autism and Asperger Diagnostic Scale 14-item version (RAADS-14), and AdAS Spectrum. Results The AdAS Spectrum demonstrated excellent internal consistency for the total score (Kuder–Richardson's coefficient=.964) as well as for five out of seven domains (all coefficients>.80) and sound test–retest reliability (ICC=.976). The total and domain AdAS Spectrum scores showed a moderate to strong (>.50) positive correlation with one another and with the AQ and RAADS-14 total scores. ASDc subjects reported significantly higher AdAS Spectrum total scores than both FED ( p <.001) and CTL ( p <.001), and significantly higher scores on the Childhood/adolescence , Verbal communication , Empathy, Inflexibility and adherence to routine , and Restricted interests and rumination domains (all p <.001) than FED, while on all domains compared to CTL. CTL displayed significantly lower total and domain scores than FED (all p <.001). A significant effect of gender emerged for the Hyper– and hyporeactivity to sensory input domain, with women showing higher scores than men ( p =.003). A Diagnosis* Gender interaction was also found for the Verbal communication ( p =.019) and Empathy ( p =.023) domains. When splitting the ASDc in subjects with one symptom criterion (ASD 1 ) and those with a ASD, and the FED in subjects with no ASD symptom criteria (FED 0 ) and those with one ASD symptom criterion (FED 1 ) , a gradient of severity in AdAS Spectrum scores from CTL subjects to ASD patients, across FED 0 , ASD 1 , FED 1 was shown. Conclusions The AdAS Spectrum showed excellent internal consistency and test–retest reliability and strong convergent validity with alternative dimensional measures of ASD. The questionnaire performed differently among the three diagnostic groups and enlightened some significant effects of gender in the expression of autistic traits. [ABSTRACT FROM AUTHOR]