Protocol for the Measurement of Combined Serum and Urine Cystatin C, Neutrophil Gelatinase-Associated Lipocalin, and Retinol-Binding Protein in Early Diabetic Nephropathy Diagnosis

This study aimed to validate latex immunoturbidimetric assays for Cystatin C, neutrophil gelatinase-associated lipocalin, and retinol-binding protein to evaluate their combined diagnostic utility in early diabetic nephropathy. In addition, a comparison was made between the diagnostic efficacy of serum and urine samples. The study comprised 100 patients with early DN and 100 control subjects. Serum and urine levels of Cys-C, NGAL, and RBP were measured using a automated biochemical analyzer. Method verification demonstrated that all assays were performed in accordance with the manufacturer's specifications, exhibiting high levels of accuracy, precision, and correlation with reference methods. Serum Cys-C, NGAL, RBP-S and their urinary counterparts were significantly higher in the DN group (P < 0.05). The combined detection of urinary Cys-C, NGAL, and RBP-U yielded an area under the curve of 0.748, with a sensitivity of 72.0% and a specificity of 75.0%. This non-invasive urinary approach demonstrated superior diagnostic efficacy compared to both single-marker tests and the serum panel. In conclusion, the assays can be considered reliable. The combined detection of Cys-C, NGAL, and RBP in urine offers superior diagnostic value for early diabetic nephropathy compared to serum analysis, thereby providing an optimized and non-invasive diagnostic strategy.