Determinants of Access to Autism Spectrum Disorder Diagnostic Services: A Systematic Review and Meta-Analysis of Factors Associated With Diagnostic Completion, Diagnostic Pathways, and Timely Diagnosis

This systematic review and meta-analysis aimed to estimate overall access to autism spectrum disorder (ASD) diagnostic services and identify the key determinants and sources of heterogeneity across the diagnostic pathway using frequentist and Bayesian approaches.A systematic search of MEDLINE/PubMed, Embase, Scopus, Web of Science, Global Health, and grey-literature sources was conducted for studies published between January 2004 and December 2024.Studies examining ASD diagnostic completion, diagnostic pathways, diagnostic timeliness, or barriers and facilitators to diagnostic access were included.A total of 22 unique studies were synthesized, with nine studies contributing 23 quantitative effect estimates.Random-effects models using restricted maximum-likelihood (REML), Bayesian hierarchical analysis, metaregression, subgroup analyses, and sensitivity analyses were conducted following PRISMA guidelines.The pooled random-effects estimate indicated that approximately three-quarters of observed ASD diagnostic outcomes represented successful access (74.1%, 95% CI: 65.8-81.1%).Heterogeneity was substantial (I²=88.4%)with a wide 95% prediction interval (32.8%-94.4%).Bayesian analysis produced a comparable pooled estimate (73.3%, 95% CrI: 65.3%-80.2%),with high residual heterogeneity (I²≈87.5%).By outcome domain, pooled successful outcomes were highest for diagnostic pathways (89.3%, 95% CI: 70.1%-96.7%),followed by timely diagnosis (76.3%, 95% CI: 62.9%-86.0%),and lowest for diagnostic completion (67.1%, 95% CI: 61.8%-72.0%);differences between domains were borderline significant (Qₘ=5.98,p=0.050).Timely diagnosis demonstrated particularly high heterogeneity (I²=91.2%),whereas diagnostic completion showed moderate heterogeneity (I²=40.6%).Across determinant domains, pooled estimates were highest for child developmental/neurobehavioral factors (79.5%), followed by family, socioeconomic and perceptual factors (74.2%), intervention/care-navigation factors (68.0%), and provider/clinical recognition factors (63.6%).Meta-regression identified significant variation by determinant category (Qₘ=13.48,p=0.004) and effect measure (Qₘ=7.81,p=0.020), whereas age group and geographic region were not significant moderators.Family/socioeconomic factors (B=2.703,95% CI: 0.661-4.744;p=0.009) and child developmental/neurobehavioral factors (B=1.516,95% CI: 0.047-2.985;p=0.043) were associated with larger effects.Publication bias assessments suggested potential small-study effects, although sensitivity analyses excluding two influential estimates produced a stable pooled outcome of 77.1% (95% CI: 71.6-81.9%),supporting robustness of the principal finding.Overall, approximately 74% of observed ASD diagnostic outcomes represented successful access, but substantial heterogeneity indicates that diagnostic access is highly context-dependent.Family and socioeconomic circumstances and child developmental characteristics emerged as important determinants, while provider-related effects were more heterogeneous.The findings support interventions spanning the entire diagnostic pathway, including developmental surveillance, screening, referral coordination, family navigation, provider capacity, specialist availability, and mechanisms to improve completion of diagnostic assessment, with further longitudinal and implementation research particularly needed in low-and middle-income countries.