Factors Associated with Extreme Waiting Times for Child Developmental Diagnostic Assessments: A Mixed-Methods Service Evaluation in Sydney

Background: Previous Australian child developmental assessment service (CDAS) cohorts have identified a small proportion of children experiencing extreme waiting times (≥730 days) for standardised developmental assessments. However, factors associated with these extreme waits remain poorly understood. Aim: To examine factors associated with a priori defined extreme waiting times (≥730 days) by comparing affected children with the remaining CDAS cohort, while validating administrative data and exploring caregiver experiences. Methods: An embedded mixed-methods service evaluation was undertaken. Quantitative data were extracted from the CDAS database and electronic medical records for children assessed between October 2022 and September 2024. Children with validated extreme waiting times were compared with the remaining CDAS cohort. Records initially identified as waiting ≥730 days underwent manual validation of referral and assessment dates. Caregivers of children with validated extreme waits were purposively recruited for semi-structured interviews. Fifteen caregivers participated; twelve families did not respond. Interview transcripts underwent thematic analysis. Results: Of 67 children initially identified as waiting ≥730 days, 20 (29.9%) were excluded following manual validation because of administrative recording inaccuracies, leaving 47 children with confirmed extreme waiting times. Of these, 46 completed CDAS assessment and had available validated waiting-time data, with a median wait of 859 days (IQR 775–932), compared with 254 days (IQR 111–355) in the comparison cohort of 2039 other children. Children in the extreme-wait cohort were more often from culturally and linguistically diverse backgrounds (76.6% vs. 53.5%; p = 0.002), required interpreters more frequently (23.4% vs. 9.0%; p = 0.003), and were more likely to be referred by paediatricians (89.4% vs. 74.1%; p = 0.017). Over one-third (n = 17) were prioritised for assessment within six months at triage. Review assessments accounted for 25/47 (53.2%) of the extreme-wait cohort. Caregivers described acceptance of lengthy public-sector waits, communication gaps, uncertainty regarding service pathways, reliance on interim supports, financial barriers to private assessment, and positive experiences once assessments were completed. Conclusions: Children experiencing extreme waits had distinct characteristics. Extreme waiting reflected genuine service delays, planned clinical review pathways, and administrative recording inaccuracies. Administrative data should be validated before waiting-time metrics are used for service evaluation. Improved communication, service navigation, equitable access to interim supports, and targeted service redesign may improve caregiver experiences and reduce extreme waits in publicly funded developmental assessment services.