This study evaluated the effectiveness of a short scale in screening for autism spectrum disorders (ASDs) and documented the prevalence of ASD in twins born in 1988–2000 as registered in the Danish Psychiatric Central Research Registry (DPCRR). Five child behaviour checklist (CBCL) items were used in a first-phase screening of all twins born 1988–2000, recruited from the Danish Twin Registry. In Denmark, nearly all psychiatric diagnoses are reported to DPCRR, and the two registers were linked for validation purposes. Parents of >16,000 twins responded (68.4%); among the twins, 108 were registered with ASD. The optimal cut-off score of two out of ten yielded a sensitivity of 79.6% and a specificity of 81.4%. The registry linkage identified 176 twins with ASD (point prevalence = 0.72%). This study demonstrates that it is feasible to screen large populations for ASD with a 5-item questionnaire. The prevalence of registered ASD in twins corresponds to recent population-based studies in singletons. 1. Introduction Autism spectrum disorders (ASDs) represent the expanded concept of autism. ASD overlaps the category of pervasive developmental disorders (PDDs) in the ICD-10 International Classification of Disease [1] and in the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR) [2]. ASDs comprise autism, Asperger’s syndrome (AS), and other atypical forms, such as PDD not otherwise specified (PDD-nos) and atypical autism. The core symptom is pervasive impairment in mutual social interaction, and additional deficits reflect either verbal or nonverbal communicative impairments and/or restrictive repetitive and stereotyped patterns of behaviour, interest, and activities. Diagnosis of ASD can already be made from the age of 2-3 years, and the majority of individuals with ASD are identified before adulthood [3]. Population-based prevalence estimates reach around 60–70 per 10,000 [4] though estimates higher than 1% have been reported too [5, 6]. In epidemiological studies of ASD, questionnaires are widely used for screening [5, 6]. Participation rates in population studies are often low, and there seems to be an inverse relationship between the length of a questionnaire and the response rate in population surveys [7]. To minimize attrition, epidemiologists suggest a two-phase screening procedure, with a first step of a few highly sensitive questions followed by a second step using a more specific and elaborate questionnaire [8]. This paper is based on screening data from a survey of all twins registered in the national Danish Twin Registrar (DTR), as a
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