Letter to the Editor: “Diabetic Nephropathy: Has the Time Come to Move beyond Plasma Creatinine?”

Diabetic nephropathy is the leading cause of kidney disease in patients with end stage renal disease and affects 40% of type 1 and type 2 diabetic patients. The major feature of diabetic nephropathy is an increase in urinary albumin excretion (UAE) in the absence of other renal diseases. Diabetic nephropathy is categorized into two stages: microalbuminuria (UAE >20 µg/min and ≤199 µg/min) and macroalbuminuria (UAE 200 µg/min). In patients with type 2 diabetes or features of metabolic syndrome, screening for microalbuminuria should be performed at diagnosis and yearly thereafter. For type 1 diabetes, screening should be performed five years after diagnosis and then yearly. However, some diabetic patients were found to have normal UAE and a significant reduction in GFR.[[1–3]]

The estimation of GFR is an indispensable tool to determine renal function in not only diabetic patients, but in the general population as well. In patients with type 2 diabetes in the NHANES III (Third National Health and Nutrition Examination Survey), low GFR (<60 mL · min⁻¹ · 1.73 m⁻²) was present in 30% of patients in the absence of micro- or macroalbuminuria and retinopathy.[[1]] There are some patients with either type 1 or type 2 diabetes who have decreased glomerular filtration rate (GFR) in the presence of normal UAE.[[2],[3]] In patients with type 1 diabetes, this phenomenon seems to be more common among female patients with longstanding diabetes, hypertension, and/or retinopathy.[[2]] These studies indicate that normoalbuminuria does not protect from a decrease in GFR in type 1 and type 2 diabetic patients. Therefore, GFR should be routinely estimated and UAE routinely measured for a proper screening of diabetic nephropathy. The recommended equation for GFR estimation by the National Kidney Foundation is the Modified Diet in Renal Disease (MDRD) equation: GFR (ml · min⁻¹ · 1.73 m⁻²) = 186 × [serum creatinine (mg/dL)^−1.154 × age (years)^−0.203 × (0.742 if female) × (1.210 if African American)].[[4]] The MDRD is readily accessible online at http://www.kidney.org/klsprofessionals/gfr_calculator.cfm. The reference range of GFR values in young individuals is from 80 to 130 ml · min⁻¹ · 1.73 m⁻², declining at #10 ml · min⁻¹ · decade⁻¹ after 50 years of age.[[5]]

On the other hand, in microalbuminuric patients, GFR remained stable in a subset of patients that showed a rapid decline in GFR levels.[[6]] Patients with a more rapid GFR decline usually have more advanced diabetic glomerulopathy and worse metabolic control. Early referral to a nephrologist should be considered, especially when GFR reaches 30 ml·min⁻¹·1.73m⁻².[[6]] Therefore, GFR is the best parameter of overall kidney function and should be measured or estimated in micro- and macroalbuminuria diabetic patients.