Background: IDH mutations are common in low-grade gliomas and confer significantly improved prognosis. IDH catalyzes the oxidative decarboxylation of isocitrate to α-ketoglutarate, and subsequently to the oncometabolite 2HG. Mutant IDH leads to preferential accumulation of the R relative to the S enantiomer of 2HG. We analyzed the ratio of R to S enantiomers (rRS) in glioma tissues and matched serum samples, and correlated findings with IDH status, 1p19q codeletion status and survival. Methods: Fresh frozen glioma tissues and matched serum samples were obtained from the University of Toronto Brain Tumor Bank. IDH status was determined by immunohistochemistry and confirmed by 450K methylation profile or direct sequencing. 1p19q codeletion status was determined by loss-of-heterozygosity PCR analysis. R-2HG and S-2HG levels were quantified using HPLC tandem mass spectrometry coupled with a CHIROBIOTIC column. Results: Glioma tissues from 70 patients were analyzed – 52 with IDH MUT and 18 WT. 30 had matched serum samples. Using glioma tissues, rRS clearly distinguished MUT vs WT (median 574 vs 1.3, p < 1x10^-9) with only three outliers. In contrast, rRS was not elevated in serum samples (median 1.5 vs 1.2, p = 0.13). Overall survival (OS) was significantly longer for MUT vs WT (median 178 vs 33 months, p < 1×10^-7). Stratifying MUT by tissue rRS, median OS was 197, 178, 178 and 122 months for lowest to highest quartiles of rRS respectively. 1p19q codeletion status and tumor latency did not explain this trend, given rRS was similar in codeleted vs non-codeleted MUT, and similar in MUT operated < 3 vs ≥3 months from diagnosis. Progression-free survival results corresponded to OS. Conclusions: rRS from glioma tissues effectively differentiated MUT vs WT, whereas serum samples were unreliable. Unlike current methods, tissue rRS enables real-time determination of IDH status, and thus may guide clinical practice such as extent of surgical resection intraoperatively and upfront selection of adjuvant therapy. rRS potentially stratifies survival within MUT patients, providing detailed correlative information.