Elevated baseline glomerular filtration rate (GFR) is independently associated with a more rapid decline in renal function of patients with type 1 diabetes

Aims: Renal hyperfiltration is observed prior to the development of diabetic kidney disease (DKD) in patients with type 1 diabetes (T1DM); however its significance remains uncertain. Longitudinal data were used to investigate the association between measured baseline glomerular filtration rate (GFR) and renal function decline in patients with T1DM. 

Methods: This study included 142 adult patients with T1DM and ≥ 2 measurements of glomerular filtration rate (mGFR; determined by diethylene-triamine-penta-acetic acid plasma clearance). Median follow up was 19 years. Patients were stratified by baseline mGFR quartile. The relationship between baseline mGFR and rate of renal function decline was assessed using random-effect generalized least squares regression, adjusted for age, duration of diabetes, HbA1c, blood pressure, renin-angiotensin-aldosterone system inhibitor therapy, LDL and BMI. 

Results: The average rates of decline in renal function for the 2^nd (baseline mGFR: 96.4-112.6 ml min- ¹ 1.73 m- ²), 3^rd (baseline mGFR: 112.6-125.5 ml min- ¹ 1.73 m- ²) and 4^th quartiles (baseline mGFR >125.5 ml min- ¹ 1.73 m- ²) were significantly faster than the first quartile (baseline mGFR: 60.9-96.4 ml min- ¹ 1.73 m- ²). In further detail, the average rates of decline in the 2^nd (rate of decline 1.25 ml min- ¹ 1.73 m- ² per year, 95% CI: 0.97; 1.52, p=0.008), 3^rd (rate of decline 1.35 ml min- ¹ 1.73 m- ² per year, 95% CI: 1.08; 1.62, p= 0.001) and 4^th quartiles (rate of decline 1.6 ml min- ¹ 1.73 m- ² per year, 95% CI: 1.34, 1.88, <0.0001) were significantly faster when compared to the first quartile (rate of decline 0.67 ml min- ¹ 1.73 m- ² per year, 95% CI: 0.37; 0.96). Sub-analysis of quartile 4 revealed higher HbA1c measurements throughout follow-up in those with rapid mGFR decline (> 3.0 ml min^{- 1} 1.73 m^{- 2}/year). 

Conclusions: In patients with T1DM, higher baseline mGFR is associated with more rapid mGFR decline. Patients with high baseline mGFR who developed rapid mGFR decline had higher HbA1c measurements throughout the study. These findings are consistent with the concept that poor glycaemic control over time may be a determining factor for the rapid renal function decline observed in some hyperfiltering patients.