Standardization of GFR by BSA can underestimate kidney function in overweight individuals. Standardization of GFR by ECV, rather than BSA, may be a way to avoid measurement bias. Our aim was to compare the distribution and agreement of GFR measures in people with type 1 diabetes at risk of kidney disease, and to determine associations with clinical hemodynamic measures. In this exploratory analysis of the PERL clinical trial of allopurinol, the distribution and agreement of 4 measurements for GFR were compared in 542 participants with eligible data. GFR was measured using gold-standard iohexol plasma clearance procedures, and ECV was calculated using the Jødal-Bröchner-Mortensen method, which estimates ECV using results from clearance studies. GFR was standardized using ECV corrected for 14 L of fluid volume rather than 1.73 m2 of BSA. Agreement between measurements was assessed by Bland-Altman and Spearman correlation analyses. The trial primary outcome was re-analyzed using ECV-adjusted GFR. Mean age and BMI were 51±years and 29.5±6.0 kg/m2. The distribution of normalized GFR measures overlapped and agreement was strong; GFR/ECV overestimated GFR/BSA by 2.9 numerical units (mean 70.5 ml/min/14 L vs. 67.6 ml/min/1.73 m2) . Heart rate was associated with unstandardized GFR (r=-0.10, p=0.023) . Associations between diastolic blood pressure and all measures of GFR were observed (r ranged from 0.17-0.18; p-values Disclosure L.Lovblom: None. R.Pop-busui: Advisory Panel; Averitas Pharma, Inc., Boehringer Ingelheim International GmbH, Nevro Corp., Novo Nordisk, Reata Pharmaceuticals, Inc., Regenacy Pharmaceuticals, Inc. S.Polsky: Advisory Panel; Medtronic, Other Relationship; diaTribe, Research Support; Dexcom, Inc., Eli Lilly and Company, Leona M. and Harry B. Helmsley Charitable Trust, Medtronic, National Institute of Diabetes and Digestive and Kidney Diseases, Sanofi-Aventis U.S. A.B.Karger: Consultant; Roche Diagnostics, Research Support; Kyowa Kirin Co., Ltd., Siemens. A.Galecki: None. B.A.Perkins: Advisory Panel; Abbott Diabetes, Insulet Corporation, Sanofi, Board Member; Novo Nordisk, Other Relationship; Abbott Diabetes, Insulet Corporation, Medtronic, Novo Nordisk, Research Support; BMO Bank of Montreal, Novo Nordisk. D.Cherney: Other Relationship; AbbVie Inc., AstraZeneca, Bayer AG, Boehringer Ingelheim International GmbH, Janssen Research & Development, LLC, Lilly, Maze, BMS, CSL-Behring, Merck, Otsuka, Novartis and Novo-Nordisk , Mitsubishi Tanabe Pharma Corporation, Sanofi, Research Support; Boehringer Ingelheim-Lilly, Merck, Janssen, Sanofi, AstraZeneca and Novo-Nordisk. S.O.Lanctot: Employee; Medtronic. D.M.Maahs: Advisory Panel; Abbott Diabetes, Eli Lilly and Company, Medtronic, Novo Nordisk, Sanofi, Consultant; Aditx Therapeutics, Inc., Biospex. P.Bjornstad: Advisory Panel; AstraZeneca, Bayer AG, Boehringer Ingelheim International GmbH, Horizon Therapeutics plc, LG Chem, Lilly, Novo Nordisk, Consultant; AstraZeneca, Bristol-Myers Squibb Company. M.Mauer: None. L.Caramori: Advisory Panel; Bayer AG, Consultant; AstraZeneca, Boehringer Ingelheim International GmbH, Novo Nordisk, Research Support; Bayer AG, Novartis AG. S.Rosas: Advisory Panel; AstraZeneca, Teladoc Health, Other Relationship; Bayer AG, Research Support; AstraZeneca, Bayer AG. P.Rossing: Consultant; Astellas Pharma Inc., AstraZeneca, Bayer AG, Gilead Sciences, Inc., Merck Sharp & Dohme Corp., Novo Nordisk A/S, Sanofi, Speaker's Bureau; Eli Lilly and Company. K.R.Tuttle: Advisory Panel; Boehringer Ingelheim International GmbH, Gilead Sciences, Inc., Consultant; AstraZeneca, Eli Lilly and Company, Research Support; Bayer AG, Goldfinch Bio, Inc., Novo Nordisk, Travere. Funding Funded by the National Institute of Diabetes and Digestive and Kidney Diseases and others; PERL ClinicalTrials.gov number, NCT02017171