Aims Improved early diagnostic methods are needed to identify risk for kidney disease in people with type 1 diabetes. We hypothesized that glomerular filtration rate (GFR) measured by iohexol clearance in dried blood spots (DBS) on filter paper would be comparable to plasma (gold-standard) and superior to estimated GFR (eGFR) and, second, that adjustment for ambient blood glucose would improve accuracy and precision of GFR measurement. Methods GFR was measured by iohexol clearance in plasma, DBS, and as estimated by the CKD-Epidemiology Collaboration equations in 15 adults with type 1 diabetes at two visits, one euglycemic and one hyperglycemic. Results GFR measured by DBS was more comparable and less biased than GFR cystatin C, serum creatinine, and both combined. GFR was higher during hyperglycemia. Correction for between visit glycemia statistically significantly reduced bias and mean squared error for GFR measured by DBS as compared to gold-standard during euglycemia. Conclusions Iohexol clearance measured with DBS performed better than eGFR methods. Correction for ambient blood glucose improved precision and accuracy of GFR measurement. This method is more convenient than the gold-standard GFR method and may improve screening and diagnostic capabilities in people with type 1 diabetes, especially when GFR is >60ml/min/1.73m 2 .
