Autism spectrum disorder (ASD) is commonly diagnosed after the optimal window for early neuroplasticity, despite validated behavioural instruments. These tools remain vulnerable to rater bias, sex differences and service delays, motivating interest in molecular biomarkers. This narrative review synthesises transcriptomic evidence across brain, blood and saliva in ASD, mapping convergent synaptic, immune and mitochondrial axes, contrasting bulk, single-nucleus and long-read RNA-seq technologies, and summarising the diagnostic performance of existing blood and salivary panels in children. We integrate these data with behavioural scales, Standards for Reporting of Diagnostic Accuracy (STARD)-based reporting requirements and ethical, regulatory and economic considerations for clinical implementation. Current panels show promising but preliminary accuracy and should be viewed as adjuncts to behavioural assessment; rigorous multicentre, prospective validation is required before routine use.