The Known Unknowns: An Overview of the State of Blood-based Protein Biomarkers of Mild Traumatic Brain Injury.

Blood-based protein biomarkers have revolutionized several fields of medicine by enabling molecular level diagnosis, as well as monitoring disease progression and treatment efficacy. Traumatic brain injury (TBI) so far has benefitted only moderately from using protein biomarkers to improve injury outcome. Due to its complexity and dynamic nature, TBI, especially its most prevalent mild form (mTBI), presents unique challenges toward protein biomarker discovery and validation as blood is frequently obtained and processed outside of clinical laboratory (e.g., athletic fields, battlefield) under variable conditions. As it stands, the field of mTBI blood biomarkers faces a number of outstanding questions. Do elevated blood levels of currently used biomarkers, UCH-L1, GFAP, NFL and tau/p-tau truly mirror the extent of parenchymal damage? Do these different proteins represent distinct injury mechanisms? Is the blood brain barrier a "brick wall"? What is the relationship between intra vs extra cranial values? Does prolonged elevation of blood levels reflect, de novo release or extended protein half-lives? Does biological sex affect the pathobiological responses after mTBI and thus blood levels of protein biomarkers? At the practical level, it is unknown how preanalytical variables - sample collection, preparation, handling and stability affect the quality and reliability of biomarker data. The ever-increasing sensitivity of assay systems, the lack of quality control of samples combined with the almost complete reliance on antibody-based assay platforms represent important unsolved issues as false negative results can lead to false clinical decision making and adverse outcomes. This article serves as a commentary on the state of mTBI biomarkers and the landscape of significant challenges. We highlight and discusses several biological and methodological "known unknowns" and close with some practical recommendations.