Associations of serum interleukin-6 and glutamate with irritability severity in autism spectrum disorder: a cross-sectional biomarker-behavior study

Background Irritability and associated aggressive behaviors are common and impairing features of autism spectrum disorder (ASD). Immune dysregulation involving interleukin-6 (IL-6) and alterations in glutamatergic signaling have been implicated in ASD; however, their associations with irritability severity remain insufficiently characterized. Methods This cross-sectional study included 60 children and adolescents with DSM-5–diagnosed ASD. Irritability was assessed using the 15-item Irritability/Agitation subscale of the Aberrant Behavior Checklist–Community version (ABC-C–I). Based on the sample median of 19.5, participants scoring ≥ 20 were classified as having high irritability ( n = 30), and those scoring ≤ 19 as having low irritability ( n = 30). Serum IL-6 was measured using an enzyme-linked immunosorbent assay, and glutamate was measured using a colorimetric assay. Group comparisons, correlation analyses, and multivariable linear regression models were performed. Results The high-irritability group had higher serum IL-6 (3.40 ± 1.28 versus 2.20 ± 0.62 pg/mL; p < 0.001; Hedges’ g = 1.18) and glutamate concentrations (56.81 ± 6.99 versus 49.86 ± 6.43 µmol/L; p < 0.001; Hedges’ g = 1.02). ABC-C–I scores were positively correlated with IL-6 ( r = 0.50, p < 0.001) and glutamate ( r = 0.39, p = 0.002). In the adjusted model, IL-6 (B = 2.16, p = 0.015) and glutamate (B = 0.25, p = 0.004) were independently associated with ABC-C–I scores. After participants reporting recent infection were excluded, the IL-6 association remained significant ( p = 0.002), whereas the glutamate association did not ( p = 0.206). Additional analyses indicated that the glutamate findings were sensitive to how measurements below the assay’s detection limit were handled. Conclusion Higher serum IL-6 and glutamate concentrations were associated with greater irritability severity in this cross-sectional sample. The IL-6 finding was more robust across sensitivity analyses, whereas the glutamate finding requires cautious interpretation. Longitudinal studies are needed to establish temporal direction and clinical relevance.