Despite their range and complexity, adherence to Scottish Intercollegiate Guidelines Network guideline for the diagnosis and assessment of autism spectrum disorders (ASD) was shown to be high within child development and specialist diagnostic clinics serving a geographical cohort of children diagnosed under the age of 7 years. A retrospective analysis of comprehensive clinical notes demonstrated that the recommended discretionary use of structured history instruments was increased after medical training (p = 0.003). 56% (51/90) of children received the diagnosis of ASD at their initial specialist appointment. 51% underwent the recommended discretionary structured observational instrument. This further assessment was more likely to be required for older children in the reaudited group (p = 0.001). The implications for service capacity planning when delivering best practice recommendations are discussed.