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

Background: Autism spectrum disorder (ASD) is a lifelong neurodevelopmental condition for which timely diagnosis is critical to early intervention, family support, and equitable access to care. However, substantial disparities in access to ASD diagnostic services persist across socioeconomic, geographic, clinical, and health-system contexts. This systematic review and meta-analysis synthesized evidence on determinants of access across the ASD diagnostic pathway, from recognition and referral to diagnostic completion and timely diagnosis. Methods: We systematically searched MEDLINE/PubMed, Embase, Scopus, Web of Science, Global Health, and grey-literature sources for studies published between January 2004 and December 2024. Eligible studies examined determinants of ASD diagnostic completion, diagnostic pathways, diagnostic timeliness, or barriers and facilitators to diagnostic access. Two reviewers independently extracted data and assessed methodological quality using the Mixed Methods Appraisal Tool (MMAT). Quantitatively comparable estimates were synthesized using random-effects models with restricted maximum likelihood estimation. Heterogeneity was assessed using Cochran's Q, I2, tau2, and 95% prediction intervals. Pre-specified subgroup analyses, meta-regression, sensitivity analyses, funnel-plot assessments, and Bayesian random-effects analyses were undertaken. Results: The search identified 4,899 records; after removal of 537 records without associated data, 4,362 records underwent title/abstract screening. 3,800 records were excluded, 562 reports were sought for retrieval, and 450 full-text reports were assessed after 112 could not be retrieved. Ultimately, 22 unique studies met the inclusion criteria. Nine unique studies contributed 23 quantitative effect estimates, while the remaining studies contributed to the narrative synthesis. The evidence covered socioeconomic, geographic, family, communication, screening, child developmental, provider, and health-system determinants. The overall random-effects meta-analysis yielded a pooled diagnostic access outcome of 74.1% (95% CI 65.8-81.1%), with substantial heterogeneity (Qe=209.95, p<0.001; I2=88.4%, 95% CI 79.1-94.4%; tau2=0.691) and a wide 95% prediction interval of 32.8-94.4%. Bayesian analysis produced a highly concordant pooled estimate of 73.3% (95% CrI 65.3-80.2%), with I2=87.5% and tau=0.833, and satisfactory MCMC convergence (R-hat=1.000). 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%) (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, frequentist pooled estimates were 79.5% for child developmental/neurobehavioral factors, 74.2% for family/socioeconomic/perceptual factors, 68.0% for intervention/care-navigation factors, and 63.6% for provider/clinical recognition factors. Bayesian estimates were 76.7% (BF=53.76), 72.9% (BF=226.32), 64.3% (BF=25.60), and 53.7% (BF=0.684), respectively. Meta-regression indicated that determinant category (Qm=13.48, p=0.004) and effect measure (Qm=7.81, p=0.020) significantly explained between-study variation, whereas age group (p=0.203) and geographic region (p=0.453) did not. Family/socioeconomic factors had significantly larger effect sizes (B=2.703, 95% CI 0.661-4.744; p=0.009), as did child developmental/neurobehavioral factors (B=1.516, 95% CI 0.047-2.985; p=0.043). Potential small-study effects were detected by two of three asymmetry tests, although the Rosenthal fail-safe N was 1,723. Trim-and-fill identified seven potentially missing estimates, with an adjusted pooled effect of 68.4% (95% CI 27.7-109.1%). Importantly, exclusion of two influential outlying estimates produced a pooled outcome of 77.1% (95% CI 71.6-81.9%), indicating that the principal finding was robust. Conclusions: Approximately three-quarters of observed ASD diagnostic outcomes represented successful access, but the substantial heterogeneity indicates that diagnostic access is highly context-dependent. Families were more likely to successfully navigate diagnostic pathways than to complete diagnostic assessment, while timely diagnosis showed the greatest variability across settings. Family and socioeconomic circumstances and child developmental characteristics emerged as particularly important determinants, whereas provider-related effects were more heterogeneous and uncertain. Improving equitable ASD diagnosis requires interventions spanning the entire diagnostic pathway, including developmental surveillance, screening, referral coordination, family navigation, provider capacity, specialist availability, and mechanisms to ensure completion of diagnostic assessment. Greater longitudinal and implementation research is particularly needed in low- and middle-income countries, where diagnostic infrastructure and specialist capacity remain limited.