2077 Background: Diagnosis and treatment decisions for CNS tumors rely upon imaging with subsequent histological and genomic analyses of tissue to identify appropriate courses of therapy. Genomic profiling of tumor tissue for informative alterations is used to guide treatment, however contemporaneous tissue can sometimes be unavailable, inconclusive or overlook possible actionable alterations given the high amount of spatial heterogeneity in certain tumor types such as glioblastomas GBMs. Liquid biopsies may pose a possible solution, as they are readily accessible and provide a holistic snapshot of the tumoral genomic landscape. However, detection of ctDNAs from CNS tumors is impeded by its inherently low shedding biology and poor permeability across the BBB resulting in low plasma variant allele fractions (VAFs), with alteration detection rates ranging 27-55%. Thus, current assays do not yet have the sensitivity to be clinically useful in aiding decision making for CNS tumors. Methods: 62 cases of primary CNS tumors were submitted commercially for assessment with Northstar Select, a tumor-naive, plasma-based ctDNA liquid biopsy assay. Cases were obtained from a single clinic dating from January-December 2023. Results: See table. Conclusions: There remains an unmet clinical need for a highly sensitive liquid biopsy assay for use in determining the genomic landscape of CNS tumors to aid in clinical decision making. Current assays do not have a low enough limit of detection (LoD) to accurately detect ctDNAs at low VAFs. Herein, we demonstrate that Northstar Select has a detection rate of 88.7% in all cases and 91.4% in GBMs, nearly doubling current rates. This is due to the assay’s uniquely low LoD of 0.13-0.16% VAF, which is below the 0.17% median VAF demonstrated across CNS tumors in this study, allowing for more low-abundance alterations to be detected. Given the increased total variant detection rate, nearly 50% of which were actionable when alterations were detected, these data support the clinical utility of Northstar Select to inform clinical decisions either as a complement to imaging and tissue analyses or independently when tissue biopsies are infeasible. These data further suggest that the clinical utility of Northstar Select may be expanded into metastatic brain disease and other low ctDNA shedding tumors. [Table: see text]