Boundaries, transitions:Autism Spectrum Disorder in adolescents and adults

There has been a substantial increase in referrals of adults for assessment of Autism Spectrum Disorder (ASD ) in the last two decades. Diagnostic evaluation of autism is a complicated process; symptom presentation may vary widely between and within individuals over time and the amount of psychiatric syndromes with an overlap with symptoms of ASD increases towards adulthood. There is a need of valid instruments to support the diagnostic process and more knowledge about the course of autistic symptoms over time. The first part of this thesis focuses on the predictive validity of existing self-report ASD questionnaires, and the psychometric properties of a new instrument, the Adult Social Behavior Questionnaire (ASBQ), a multidimensional ASD measure containing both a self-report version and a version to be completed by someone close. In the second part the manifestation and course of autistic and non-autistic symptoms in young adults who received a diagnosis of milder forms of ASD during childhood were studied.Main findings are that (1) Existing ASD questionnaires lack the predictive validity needed for a screening measure in clinical practice, which may be is too demanding in case of ASD based on only a questionnaire (2) The ASBQ shows good reliability and validity and support diagnostic decision making; (3) Most individuals with milder ASD problems in childhood do not meet sufficient criteria for an ASD classification in adulthood (but should still be monitored for psychiatric problems) (4)Sex differences in ASD were less prominent than was expected on the basis of recent literature