Page 1 of 21 © Annals of Translational Medicine. All rights reserved. Ann Transl Med 2019;7(23):792 | http://dx.doi.org/10.21037/atm.2019.11.53 Review Article Emerging biomarkers in autism spectrum disorder: a systematic review Richard E. Frye 1,2 , Sarah Vassall 1 , Gurjot Kaur 1 , Christina Lewis 1 , Mohammand Karim 1,2 , Daniel Rossignol 3 1 Barrow Neurological Institute at Phoenix Children’s Hospital, Phoenix, AZ, USA; 2 Deparment of Child Health, University of Arizona College of Medicine, Phoenix, AZ, USA; 3 Rossignol Medical Center, Aliso Viejo, CA, USA Contributions: (I) Conception and design: RE Frye; (II) Administrative support: S Vassall, G Kaur, C Lewis; (III) Provision of study materials or patients: None; (IV) Collection and assembly of data: S Vassall, G Kaur, C Lewis, M Karim; (V) Data analysis and interpretation: RE Frye, D Rossignol; (VI) Manuscript writing: All authors; (VII) Final approval of manuscript: All authors. Correspondence to: Richard E. Frye. Barrow Neurological Institute at Phoenix Children’s Hospital, Phoenix, AZ 85016, USA. Email: rfrye@phoenixchildrens.com. Abstract: Autism spectrum disorder (ASD) affects approximately 2% of children in the United States (US) yet its etiology is unclear and effective treatments are lacking. Therapeutic interventions are most effective if started early in life, yet diagnosis often remains delayed, partly because the diagnosis of ASD is based on identifying abnormal behaviors that may not emerge until the disorder is well established. Biomarkers that identify children at risk during the pre-symptomatic period, assist with early diagnosis, confirm behavioral observations, stratify patients into subgroups, and predict therapeutic response would be a great advance. Here we underwent a systematic review of the literature on ASD to identify promising biomarkers and rated the biomarkers in regards to a Level of Evidence and Grade of Recommendation using the Oxford Centre for Evidence-Based Medicine scale. Biomarkers identified by our review included physiological biomarkers that identify neuroimmune and metabolic abnormalities, neurological biomarkers including abnormalities in brain structure, function and neurophysiology, subtle behavioral biomarkers including atypical development of visual attention, genetic biomarkers and gastrointestinal biomarkers. Biomarkers of ASD may be found prior to birth and after diagnosis and some may predict response to specific treatments. Many promising biomarkers have been developed for ASD. However, many biomarkers are preliminary and need to be validated and their role in the diagnosis and treatment of ASD needs to be defined. It is likely that biomarkers will need to be combined to be effective to identify ASD early and guide treatment. Keywords: Autism spectrum disorder (ASD); biomarkers; epigenetic; eye movement; folate; genetic; mitochondria; methylation; neuroimmune; neurophysiology; redox Submitted Oct 28, 2019. Accepted for publication Nov 05, 2019. doi: 10.21037/atm.2019.11.53 View this article at: http://dx.doi.org/10.21037/atm.2019.11.53 Introduction Autism spectrum disorder (ASD) is perhaps one of the most important medical disorders of our era because of the number of people it affects. The prevalence of ASD has increased signifcantly over the past three decades and now is estimated to affect 2% (1) or more (2) of children in the United States (US). Even more signifcant is the fact that ASD does not occur in isolation. Children with ASD require significant support from the educational, medical and social systems that results in a significant economic burden (3) which is estimated to cost the US approximately $268 billion in 2015 (4). In addition, the disability of a child creates a spillover effect, decreasing the quality of life for the entire family (5-7). One of the major limitations when identifying, diagnosing, treating and understanding ASD is the fact that ASD is exclusively defined based on the observation