Monitoring renal function in obese patients with type 2 diabetes mellitus in daily practice

Endogenous creatinine clearance (GFR(24-hr)) and the Cockcroft formula (GFR(Cockcroft)) are used for the assessment of the glomerular filtration rate (GFR) in daily practice. The influence of extreme obesity in Type 2 diabetes mellitus (T2DM) on the Cockcroft formula was evaluated.

We compared GFR as calculated by GFR(Cockcroft) to GFR as determined by GFR(24-hr) in 210 patients with T2DM and normal serum creatinine levels. The influence of body mass index (BMI) on the difference between both methods was evaluated.

GFR(Cockcroft) was an overall good predictor for GFR(24-hr), but a large individual difference between both methods was observed. A significant correlation was found between the two methods (r=0.962, p<0.001). In T2DM patients with a BMI>35 kg/m2, mean GFR(Cockcroft) was 18% greater than GFR(24-hr). For every unit BMI in this patient group the GFR(Cockcroft) increased by 7.9% relative to the GFR(24-hr).

The major limitation of this evaluation is the lack of a gold standard for GFR measurement. This evaluation of daily practice shows the GFR estimation by the Cockcroft formula and the 24-hr creatinine clearance are in agreement, although a large individual variation was found between both methods making the interchangeability questionable. In view of the influence of BMI on the Cockcroft formula, this should not be used to estimate the GFR in T2DM patients with extreme obesity.