• subthreshold
• feeding and eating disorders (FED)
• autism spectrum disorder (ASD)
• autism

Increasing epidemiological and genetic studies have raised the issue of a clinical and etiological continuity between Autism Spectrum Disorder (ASD) and autistic traits in non-clinical populations. In this perspective, still disregarded by the DSM-5, autistic traits are conceptualized as both cardinal and associated features of ASD, continuously distributed in the general population and used to characterize the so-called Broad Autism Phenotype (BAP), frequently found among relatives of people with autism. A growing body of works have also documented higher rates of mental disorders in relatives of autistic patients with respect to healthy control subjects, suggesting that autistic traits in different domains of functioning, including subclinical and atypical features, could play a significant role in raising the risk for mental disorders as well as for suicidality. At the same time, some studies have also hypothesized that a psychiatric comorbidity may happen to mask ASD, especially in subjects with normal intelligence and moderate ASD symptoms. Recent literature, in fact, has highlighted a possible link between ASD and Eating Disorders (EDs) with some authors even suggesting that autistic traits may manifest as Anorexia Nervosa in females.
In line with these studies, the presence of subclinical and often undetected forms of ASD among patients seeking for treatment for other mental disorders could be of potentially clinical significance needing an appropriate diagnosis and treatment. Consistently, increasing literature suggests the usefulness of dimensional approaches to autism. In the framework of the so-called Spectrum Project, an Italian–American collaboration research project (www.spectrumproject.org), a spectrum approach has been proposed that encompasses not only clinical features associated with the DSM mental disorders, but also isolated and atypical symptoms, as well as subthreshold symptom clusters, personality traits and behavioral manifestations that may precede, follow or be manifested in concurrence with DSM mental disorders. 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), that includes 160 items, coded in a dichotomously (yes/no), organized into seven domains.
The data were collected between Maj 2015 and April 2016 at 7 Italian University Departments of Psychiatry, coordinated by the University of Pisa: Brescia, Catania, Florence, Naples, Pavia and Siena. A total sample of 405 subjects were enrolled, in particular: 102 adults endorsing at least one DSM-5 symptom criterion for ASD (ASDc), 143 adults diagnosed with Feeding and Eating Disorders (FED) and 160 subjects with no current or lifetime mental disorders (CTL). All subjects were also administered the Structured Clinical Interview for DSM-5 Axis-I disorders (SCID-5), the Autism Quotient (AQ), the Ritvo Autism and Asperger Diagnostic Scale 14-item version (RAADS-14).
The AdAS Spectrum demonstrated excellent overall internal consistency (Kuder–Richardson’s coefficient=.964), as well as for five out of seven AdAS Spectrum domains (>.90), and sound test-retest reliability (Intraclass Correlation Coefficient=.976). The total and domain AdAS Spectrum scores showed a moderate to strong (>.50) positive correlation with one another and with the AQ and the RAADS-14 total scores. ASDc subjects reported significantly higher AdAS Spectrum total scores than both FED (p<.001) and CTL (p<.001), and reported significantly higher scores on the Childhood/adolescence, Verbal communication and Restricted interests and rumination domains (all p<.001) of the AdAS Spectrum, compared to FED, while on all the domains compared to CTL. These latter also displayed significantly lower total and domain scores than FED (all p<.001). Two Receiver Operating Characteristic (ROC) analyses found the best cut-off scores to identify either subjects endorsing only one DSM-5 ASD symptom criterion or those endorsing two of them. The two cut-off values assuring an excellent balance between sensitivity and specificity were 45 (sensitivity = 0.706, specificity = 0.769) and 60 (sensitivity = 0.971, specificity = 0.887), respectively.
In conclusion, we found excellent internal consistency and test-retest reliability, strong convergent validity with alternative dimensional measures of autism. As expected, the questionnaire performed differently among the three diagnostic groups explored.