Background:

Glomerular filtration rate (GFR) provides a valuable indicator of renal function. The gold standard for measuring GFR is inulin clearance using constant infusion and timed urine collections, however, this is a cumbersome procedure.

Aims:

To compare inulin-GFR with ^99mTc-diethylene-triamine-penta-acetic acid (^99mTc-DTPA)-GFR using single injection plasma disappearance rates in patients with type 2 diabetes, and to determine best sampling times following inulin injection. Also, to compare inulin-GFR with GFR estimates using MDRD and CKD-Epi equations.

Methods:

We compared GFR measured by different methods in 26 patients with type 2 diabetes. Plasma ^99mTc-DTPA and inulin clearance were determined by the slope intercept method using all timepoints and modified according to Brochner-Mortensen equation for one compartment model ¹ . After a bolus intravenous injection of 5g inulin, blood samples were taken at 20, 60, 120, 165, 210 and 240 minutes. Serum inulin concentration was measured by HPLC and clearance was also calculated using two sampling times as described previously ² .  

Results:

Mean ^99mTc-DTPA-GFR was 78.8mL/min/1.73m² with a range from 22 to 149ml/min/1.73m². There were 21 combinations possible using 2 blood sampling times after inulin injection to calculate GFR. Table 1 illustrates bias, accuracy and precision of GFR with accuracy >70% when compared with ^99mTc-DTPA-GFR. GFR calculated using samples at 20 and 120 minutes after inulin injection yielded the best result, with accuracy of 72% and bias of 2.4ml/min/1.73m², and this improved in the subgroup with GFR >60ml/min/1.73m². Accuracy with 2-point inulin GFR was comparable to inulin clearance calculated with slope-intercept method using all timepoints. CKD-Epi performed best overall, however, results were similar to inulin-GFR once GFR was >60ml/min/1.73m².

Conclusion:

Single injection inulin-GFR provides an efficient and simple alternative to radioisotopic methods in diabetic patients. Calculating GFR using samples 20 & 120 minutes after inulin injection gave comparable results with that of ^99mTc-DTPA-GFR, particularly in GFR above 60mL/min/1.73m². Further assessment of patients with GFR in the hyperfiltering range with this method is required.