The aim of this study was to assess the usefulness of the AQ in differentiating between adult ASD and adult ADHD and to explore whether SUD comorbidity affects the AQ scores and hence the discriminant power of the AQ when using it in patient populations with SUD comorbidity.
This study indicates that the total AQ score and most of the AQ subscale scores (except the subscale score Attention to details) are significantly and substantially higher in adults with ASD compared to adults with ADHD, and that this effect is independent from SUD status. This is an important finding because it implies that the AQ might be a valid instrument in the differential diagnosis of ASD versus ADHD. This adds to the findings in previous studies, showing that the AQ differentiates between subjects with ASD and (comorbid) obsessive compulsive disorder or social anxiety disorder (Cath et al. 2008
; Hoekstra et al. 2008
). Examination of the two AQ-factors that were proposed by Hoekstra et al. (Hoekstra et al. 2008
), shows that only the factor Social interaction accounts for the difference in AQ scores between ASD and ADHD subjects, but not the second factor Attention to details. The latter is composed of items referring to a perceptual style with a preference for details and patterns. We suggest that this scale does not discriminate between ASD and ADHD patients because it refers to a strategy for dealing with aspects of attention deficit that is common to both disorders. For example, it is known that people with ASD and ADHD present with comparable deficits in visual and auditory attention. Subjective reports from patients suggest that the attention deficit causes distress because they are overwhelmed by perceptual stimuli. The distress is said to reduce, when focusing on logical sequences such as patterns, telephone numbers or car license plates. In other words, the AQ differentiates between ASD and ADHD with respect to social interaction, as can be expected given that social interaction is the core deficit in ASD. People with ASD and ADHD show similarities, however, in the way they cope with attentional problems.
The second aim was to examine if the discriminating power of the AQ was negatively influenced if subjects have a comorbid SUD. The results show that this is only the case for the subscale Social skill in the ASD group, but not in the ADHD group. For all the other subscales, it appears that present or former SUD has no effect on the AQ scores, compared to subjects without a history of SUD. Our hypothesis is that comorbid SUD in ASD (mostly alcohol related in this group) improves or facilitates social skills and therefore social interaction. Alternatively, in people with ASD, alcohol may positively influence the subjective perception of their social skills, or cause a reduced awareness of social awkwardness, without actually improving their social functioning.
In sum, the AQ can differentiate between ASD and ADHD with statistical significance for the total AQ score and all subscales, with the exception of the subscale Attention to detail. However, when there is comorbidity with SUD the clinician should be cautious; in that case the total AQ score in patients with ASD can be attenuated by a lower score on the Social skills subscale.
The ROC analysis suggests a cut-off value of 26, which yields 73% correctly identified patients. However, with this cut-off we would wrongly classify about a third of all ASD patients and a fifth of all ADHD patients in our study group. In the general population, the ratio of ADHD versus ASD prevalence is about 7:1, unlike in our sample where this ratio was about 2:3. The values for sensitivity and specificity associated with the cut-off score of 26 are, however, by definition independent of the prevalence. This means that, given the prevalences of ASD and ADHD in the general population, this cut-off would (in a hypothetical group of 50 patients) by approximation lead to a positive identification of 4 out of every 6 patients with ASD, and 35 out of every 44 patients with ADHD (that is 77% correctly identified patients). Clinical usefulness of the AQ for differentiating between ASD and ADHD is, therefore, limited because it would lead to an unacceptable number of wrongly classified patients. This underscores the continued necessity of other methods, like thorough clinical interviews, and eliciting an accurate developmental history, for distinguishing between a diagnosis of ASD and ADHD.
The data also show that the factor score Attention to detail is similar in the ASD and ADHD groups. If we were to use the other factor score Social interaction (with 40 items composed of the 4 remaining subscales), and use the associated optimum cut-off of 21, the percentage correctly identified cases is almost the same (75%) as in the case of the total AQ score. This indicates that the discriminating effect doesn’t improve when we opt for this factor score instead of the total AQ score.
This study has both strengths and limitations. The main strengths are the relatively large sample size and the structured assessment of ASD and ADHD as well as the comorbid SUD’s and pathological gambling. The main limitations are the unknown representativeness of this treatment seeking sample with its normal IQ, the absence of another psychiatric control group, and the relatively small size of some of the diagnostic subgroups. The sex distribution was similar to that of ASD and ADHD patients in epidemiological samples. However, the number of subjects with ASD exceeded those with ADHD, whereas the prevalence of ADHD in the general population is much higher than that of ASD (Gillberg and Wing 1999
; Murphy and Barkley 1996
). A possible explanation is that the participating expert centers are used more often by referral sources when ASD is suspected than when ADHD is suspected, because ADHD is more readily diagnosed and treated in general adult psychiatric settings.
Although the group of adults with ASD shows significantly higher scores on the Autism Spectrum Questionnaire (AQ) compared to group of adults with ADHD, the clinical usefulness of the instrument for the differentiation between ASD and ADHD in individual cases is limited. The established optimum cut-off value of 26 is similar to cut-off values previously reported by others, but the percentage of correctly identified patients with ASD or ADHD is only 73%. Evidently, this self-report questionnaire cannot replace the clinical interview in disentangling ASD from ADHD.
Substance use disorder is a common comorbid problem accompanying ASD and ADHD. The results show that in case of comorbid SUD, adults with ASD still have significantly higher total AQ scores compared to adults with ADHD, even though SUD in people with ASD may present with a lower score on the subscale Social skills.