Autism spectrum disorder (ASD) is a neurodevelopmental condition with a prevalence of up to 1 out of 54 children and it is characterized by early onset of social-communication difficulties and repetitive or stereotypical behaviors . According to the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition Disorders (DSM-5) criteria, these symptoms cause “clinically significant impairment in social, occupational, and other fields of current functioning” .
The severity of ASD can vary greatly and is based on the degree to which social communication, insistence on the sameness of activities and surroundings, and repetitive patterns of behavior affect the daily functioning of the individual . The term “spectrum” refers to the wide range of symptoms, skills, and levels of disability in functioning that can occur in people with ASD. Some children and adults with ASD are fully able to perform all activities of daily living while others require substantial support to perform basic activities .
On the other hand, one of the main therapeutic objectives for individuals with ASD is autonomy in adulthood . Thus, a useful and easy-to-use tool for the assessment of the subject’s functional state is urgently needed to guide the therapeutic effort to obtain the best results.
Therefore, the assessment of functional impairment is crucial both for the diagnosis and the therapeutic approach to ASD. The concept of psychosocial functioning is complex and it integrates different areas of a person’s life, such as education, family, social life, working life, leisure and free time . ASD has been widely studied in children, but the quantity and quality of adult research is scarce [6, 7]. Valid, reliable, and sensitive outcome measures, which are fundamental for the development of an evidence-based for clinical effectiveness, are also lacking . To date, the little existing evidence points to a poor psychosocial outcome for adults with ASD, even in the less severe group .
Also, the different stages of adulthood have been studied in relation to the trajectory of people with ASD in different areas of adaptive functioning [9, 10]. However, there have been conducted very few studies that have directly compared the different stages of adult life in the ASD population. Lever et al.  studied the prevalence of psychiatric disorders in young, middle and older adults, finding a higher level of psychopathology in young and middle-aged adults when compared to older adults. These differences throughout adult life can be reflected in different levels of functionality, being able to establish different trajectory profiles according to the adult stage.
Existing measures to assess the level of functioning, such as the World Health Organization Disability Assessment Schedule II (WHO-DAS II) , the SF-36 Health Survey (SF-36) , the Global Assessment of Functioning Scale  or the Independent Living Scales , present important limitations: they are long questionnaires, most of them do not assess all areas in a person’s life, or they are not validated in adults with ASD. To assess functioning in ASD population, the Vineland Adaptive Behavior Scales, Second Edition (VABS-II)  and the Adaptive Behavior Assessment System, Second Edition (ABAS-II)  are also widely used. The VABS-II is the leading instrument for supporting the diagnosis of intellectual and developmental disabilities and the ABAS-II is a checklist of a broad range of skill areas related to development, behavior, and cognitive abilities. Although they are extensively used, they present some disadvantages, such as their length, making them difficult to apply routinely in a medical setting; moreover, they might not be reliable to assess functioning in people with ASD who do not present intellectual disability. Finally, to assess functioning in ASD and other neurodevelopmental disorders such as ADHD [18, 19], instruments without sufficient psychometric studies in these particular populations are being applied, such as Sheehan’s Disability Scale (SDS) . Although not having sufficient psychometric studies, these instruments are being used because of its easy application which makes them very useful in clinical settings.
According to this, efforts are needed to adapt the existing instruments and/or develop and evaluate new ones for assessing specific or related outcomes in adult individuals with ASD. There is a particular need for low cost, valid, and not time-consuming measures to facilitate research and use in daily routine follow-up visits. In this regard, the Functioning Assessment Short Test (FAST)  was developed for the clinical evaluation of functional impairment in patients suffering from mental disorders, evaluating different domains of functioning and it is very easy to apply (the administration only takes about 6 minutes long). The studied time frame refers to the last 15 days before assessment, and the test is a simple interview-administered instrument that requires very little time (6 min) to be applied. It comprises 24 items, which are divided into 6 specific areas of functioning: 1) Autonomy, referring to the patient’s capacity to do things alone and make their own decisions; 2) Occupational functioning, which refers to the capacity to maintain a paid job, efficiency to perform tasks at work, working in the field in which the patient was educated, and earning according to the level of the employment position; 3) Cognitive functioning is related to the ability to concentrate, perform simple mental calculations, solve problems, learn new information, and remember learned information; 4) Financial issues involve the capacity to manage finances and spend in a balanced way; 5) Interpersonal relationships refer to relations with friends and family, involvement in social activities, sexual relations, and the ability to defend ideas and opinions; 6) Leisure Time refers to the capacity to perform physical activities (sport, exercise) and the enjoyment of leisure activities. All items are rated on a 4-point scale, ranging from 0 (no difficulty) to 3 (severe difficulty). The global score is obtained by adding the scores of all the items. The higher the score, the greater the difficulties to function and greater impairment severity .
Results of previous studies using the FAST showed optimal validity and reliability properties in patients suffering from different mental health conditions. For instance, in patients with bipolar disorder, high internal consistency for the overall scale (Cronbach’s alpha of .909) as well as for all its domains have been found . In another study with patients with first psychotic episodes , an internal consistency level of .88 at baseline, .89 six months later, and .94 one year later have been reported. The FAST also showed good psychometric properties and could detect functional differences between patients with a diagnosis of schizophrenia and healthy subjects . Finally, Rotger et al.  obtained good psychometric properties, in terms of reliability and validity, in the measure of the functional level of adults with ADHD. Hence, the purpose of the present study is to evaluate whether the FAST could be a reliable and valid tool to assess the functional outcome in adults with ASD in an outpatient clinical setting and to study the differences in psychosocial functioning between younger and older adults with ASD.