Objective: The objective of the study was to detect AGE-immunoreactive proteins in urine, and to evaluate AGE excretion at various stages of diabetic nephropathy in type 2 diabetes assessed by the level of proteinuria.
Methods: AGEs were measured in 24-h urine collection of patients with normoalbuminuria (N) (n=22), microalbuminuria (Mi) (n=31), macroalbuminuria (Ma) (n=28), and overt proteinuria with elevated serum creatinine level (PC) (n=25). A competitive ELISA with polyclonal anti-AGE antibodies was used to monitor AGE excretion.
Results: Multiple comparison of urine AGE content among various stages of proteinuria showed significant differences (summary p<0.000). Fifty percent of samples from the group of normoalbuminuric, and only 15% of samples from the group of microalbuminuria patients were AGE negative. However, there was no significant difference in AGE excretion between the patients with persistent proteinuria and elevated serum creatinine, and those with macroalbuminuria (PC vs Ma, p=0.265). None of the samples from these two groups of patients with highest AGE content in 24-h urine was negative for AGE-immunoreactivity. In addition, the ratio between 24-h urinary AGEs and urinary albumin excretion was calculated to determine whether total 24-h urinary AGE content is an index of the toxic form of albumin released in the course of diabetic nephropathy. The ratio values were log-transformed and bivariate comparison showed significant differences between the N vs Mi (p=0.006) and Mi vs Ma (p=0.000) groups. However, there was no significant difference (p=0.407) between values in the Ma and PC groups of patients. Multiple stepwise regression analysis indicated a relationship of urinary AGE-immunoreactivity with creatinine clearance values (r=0.52, p<0.001).
Conclusion: The study demonstrated the presence of AGE-immunoreactivity in the urine of diabetic patients with various stages of proteinuria. Study results pointed to creatinine clearance as the main predictor of AGE excretion. Therefore, the measurement of urinary AGE appears to offer limited extra information in patients with impaired renal function.