Even in individuals who have successfully completed mainstream education, college, or university, an assessment of intellectual functioning can provide valuable information about their relative cognitive strengths and weaknesses. People with ASD often have uneven cognitive profiles and it may be useful to ascertain whether there are large discrepancies between visuospatial and verbal processing skills or between different subtests, as variability in different skill domains may help to explain everyday challenges.

A comprehensive assessment of intellectual abilities can be completed as part of the diagnostic assessment using well-established measures with adult norms. These include the Wechsler Adult Intelligence Scale (WAIS; Wechsler, 2008), the briefer Wechsler Abbreviated Scale of Intelligence (WASI; Wechsler, 1999), the Stanford Binet Scales-fifth edition (SB5; Roid, 2003), or the Kaufman Adolescent and Adult Intelligence Test (Kaufman & Kaufman, 1993). In cognitively able adults, it may also be informative to assess their executive functioning, including problem solving, response inhibition, mental flexibility, and planning. Measures that can be administered for this purpose include the Delis-Kaplan Executive Functions System (D-KEFS ; Delis, Kaplan, & Kramer, 2001) and the Wisconsin Card Sorting Test (WCST; Grant & Berg, 1981).

Many individuals with ID and ASD tend to have more uneven profiles of intellectual functioning compared to those with ID only, who often present with more uniform profiles. Thus, more detailed assessment of intellectual functioning may further aid differential diagnosis of ASD in those with ID (see Wolf & Ventola, 2014). The Stanford-Binet and Wechsler scales can be employed to assess cognitive skills in those with mild or moderate ID, but they may be less appropriate for some individuals with severe or profound ID and/or very limited or no speech. In such cases, the Leiter International Performance Scale (3rd Edition; Leiter-3; Roid et al., 2013), the Test Of Non-verbal Intelligence (TONI-4; Brown, Sherbenou, & Johnsen, 2010), or the Peabody Picture Vocabulary Test-4 (PPVT-4 ; Dunn & Dunn, 2007) can be attempted.

Many individuals with ASD present with discrepancies between their often higher intellectual skills and their comparatively reduced ability to care for themselves, communicate, or relate to others in everyday life. Such difficulties, despite at least average intellectual abilities, can contribute to adjustment or emotional difficulties or the breakdown of studying, work, living arrangements, or relationships. Thus, it may be helpful to assess adaptive functioning skills of cognitively able adults, especially those who present with or report difficulties living independently.

Measures that can be employed to evaluate adaptive behavior skills include the Vineland Adaptive Behavior Scales (VABS-II; Sparrow, Cicchetti, & Balla, 2005), the Adaptive Behavior Assessment System-II (ABAS-II; Harrison & Oakland, 2003), the Scales of Independent Behavior-Revised (SIB-R; Bruininks, Woodcock, Weatherman, & Hill, 1996), and the Waisman Activities of Daily Living Scale (Maenner et al., 2013). The adults themselves and/or their caregivers can act as informants and the measures can be administered using either a structured interview or a checklist format.

Individuals with ID and ASD present with significantly more impaired scores in the communication and social domains and higher maladaptive behaviors in the VABS when compared to adults with ID only matched for gender and IQ, but there are no differences in self-help, daily living, or gross motor skills (see Alim, Paschos, & Hearn, 2014). The measures discussed earlier for use with cognitively able adults are also appropriate for use with those with ID. In addition, the Matson Evaluation of Social Skills for Individuals with Severe Retardation (MESSIER; Matson, Carlisle, & Bamburg, 1998) may be useful for those with severe and profound ID.

Although vocabulary and grammar are likely to be commensurate with cognitive abilities in most intellectually able adults with ASD, there may be impairments in other language domains, such as in prosody (the quality and intonation of speech) and in semantic and pragmatic language (i.e., the ability to initiate, organize, structure, select, and interpret social communication through language). Paul, Landa, and Simmons (2014) recommend exploring discrepancies between higher syntax/vocabulary skills and lower pragmatic language skills, by observing the individual’s social communication in less formal, more naturalistic settings. Similarly, the American Speech-Language-Hearing Association’s 2006 guidelines in the diagnosis, assessment, and treatment of ASD across the life-span also recommend observing the individual in their natural social contexts, gathering information from the individual’s communication partners and “staging” communication contexts during the assessment to provide opportunities for communication strengths and needs to be demonstrated (Wetherby et al., 2006). The individual’s ability to initiate, understand, reciprocate, and maintain social communication through verbal and nonverbal means (i.e., gestures, speech, facial expressions) should be assessed in less formal, observational ways and documentation of the clinician’s observations and judgment is important (Wetherby et al., 2006). Observations of the adult’s social communication and conversation skills during the ADOS interactions, for example, is one useful semi-structured way to observe pragmatic language skills during clinical assessment.

In terms of selecting specific measures, very few, if any, standardized structured language tools assessing pragmatic language skills are available extending well into the adulthood years. The Woodcock Language Proficiency Battery (Woodcock, 1991) assesses oral language, vocabulary, antonyms, and synonyms, but provides little information on the more complex aspects of social language. Similarly, the PPVS and the Expressive Vocabulary Test-2nd edition (Williams, 2007) have norms up to the age of 90 years, but assess single-word receptive and expressive vocabulary, respectively. The 5th edition of the Clinical Evaluation of Language Fundamentals (CELF-5; Wiig, Semel, & Second, 2013) has an extended age range up to 22 years and may be useful in assessing social language skills in young adults. Similarly, the Test of Language Competence-Expanded (TLC-Expanded; Wiig & Second, 1989) assesses higher level functioning (i.e., understanding ambiguous language, listening comprehension, and making inferences) in young people up to the age of 18 years.

The PPVS and the EVT can assess single-word receptive and expressive vocabulary, respectively, in adults with ID and suspected ASD, but it is the assessment of the social use of verbal communication, even if this is rather limited, via direct observations and information from caregivers, that is more likely to be useful in assisting the process of differential diagnosis of ASD in adults with intellectual disabilities.

Individuals referred for suspected ASD for the first time in adulthood often initially present to professionals with mental health problems, primarily depression and anxiety (Hofvander et al., 2009; Lugnegard, Hallerback, & Gillberg, 2011). Symptoms of ADHD are also common. Although most of the existing literature has so far focused on children and adolescents, considerable efforts have been made in the last few years in order better to understand the comorbidity between ASD and other psychiatric disorders (for work with adults, see Buck et al., 2014; Joshi et al., 2013; Takara & Kondo, 2014).

A comprehensive assessment should therefore focus on identifying and understanding common emotional and behavioral difficulties, including low mood and self-esteem, anxiety, aggression towards self or others, self-neglect, and abuse (NICE, 2012). At the symptom level, the clinician needs to identify co-occurring psychiatric problems and to try to disentangle core autism from associated psychiatric symptoms as much as possible. For example, common areas of “diagnostic over-shadowing” involve social avoidance and repetitive behaviors, which could be a presenting concern in both ASD and social anxiety or OCD, respectively. Careful consideration of similarities and differences between core ASD and psychiatric symptoms is important, for example considering whether social avoidance is primarily due to a limited interest in social interactions, limited social skills, or excessive anxiety. Similarly, behaviors described as “obsessive” or “ritualistic” in ASD tend to be qualitatively different and to serve different purposes from OCD obsessions and compulsions (i.e., see Kerns & Kendall, 2013).

Few ASD-specific measures exist to assess psychiatric comorbidity , but several measures developed for adults without ASD can be used, even though their content and standardization data are not always entirely relevant for individuals with ASD. For example, the Achenbach System of Empirically Based Assessment (ASEBA; Achenbach & Rescorla, 2003), which includes self- and caregiver/informant-completed checklists for adults, can be employed. To aid differential diagnosis, the Structured Clinical Interview of DSM Disorders Clinician Version (SCID-CV; First, Spitzer, Gibbon and Williams 1996) can aid in obtaining detailed information about a range of psychiatric conditions from the adult or their caregiver/significant others. Two broad important adaptations need to be taken into consideration when such measures are used. Firstly, clinicians may need to modify the way the interview or self-rating form is presented. For example, many adults with ASD have difficulties talking about and describing emotions and it may be necessary to provide concrete definitions or explanations of items and/or to include visual aids, such as the use of an emotion “thermometer” or “volume” scale to describe emotions. Secondly, clinicians are encouraged to probe for both typical and less typical presentations of emotional difficulties, as there is evidence that people with ASD present with some symptoms which are very similar to those experienced by psychiatric populations, but also with atypical presentations of more ASD-specific fears, worries, or anxiety (i.e., Kerns et al., 2014; Ozsivadjian, Knott, & Magiati, 2012).