Measuring glomerular filtration rate by iohexol clearance on filter paper is feasible in adolescents with type 1 diabetes in the ambulatory setting

The American Diabetes Association recently recommended routine screening of glomerular filtration rate (GFR) in adolescents with type 1 diabetes, although this is not routinely performed clinically [1]. Current estimates of GFR lack precision and accuracy before stage 3 of chronic kidney disease (GFR \60 mL/min/1.73 m). Also, measurement of urinary albumin excretion, currently recommended as part of routine screening, does not directly measure kidney function. This is of particular concern in adolescents and young adults with type 1 diabetes, in whom renal hyperfiltration may promote renal injury, or rapid change in GFR may be missed due to the lack of acceptable screening methods for GFR. Perrin and Berg [2] recently reported that estimated GFR cannot accurately replace measured GFR to defect hyperfiltration in young patients with type 1 diabetes. We recently reported that iohexol clearance measured with dried blood spots (DBS) on filter paper performed better than the estimates of GFR using the CKD-EPI equations and was comparable to gold-standard iohexol plasma clearance in adults with type 1 diabetes [3]. GFR measured by iohexol clearance on DBS has also been evaluated in adults with a wide range of GFRs showing strong agreement to gold-standard measurements [4]. Standard iohexol GFR (iGFR) measurement requires more than 4 h in a research center, making it impractical for routine clinical monitoring of GFR. Here, we present data from a pilot study in youth with type 1 diabetes (n = 8, 70 % female, 17 ± 4 years, type 1 diabetes duration of 6.3 ± 5.0 years, HbA1c 8.9 ± 2.5 %) with the goal of demonstrating feasibility of translating this method to an ambulatory setting. Kidney function was measured by iGFR with DBS and estimated by Bouvet (cystatin C and creatinine-based GFR equation) and Schwartz (creatinine-based GFR equation). The participants had blood drawn, then iohexol injected intravenously prior to their regular diabetes visit and were sent home with filter paper to collect DBS at 120, 150, 180, 210 and 240 min after iohexol injection followed by mailing the filter paper back to our laboratory. There appeared to be less variability of GFR with the iGFR than the estimated GFR methods (Bouvet and Schwartz) (Fig. 1). As this study was performed on an outpatient basis to determine feasibility, we did not perform an in-patient gold-standard iGFR study to serve as a comparator for these three GFR measurements. However, our previous publication in adults with type 1 diabetes demonstrated that iGFR measured in DBS on filter paper was significantly less biased and more comparable to a gold-standard iGFR measurement than estimated GFR [3]. Moreover, using two spots at 120 and 240 min was comparable to using five spots [3], which if validated would further decrease burden to the patient and cost of this method. The adolescents also rated eight statements on a 7-point non-validated Likert scale (Fig. 2). All participants agreed or strongly agreed that the procedure was preferable Managed by Massimo Federici.