Protocol for Combined Serum and Urine Cystatin C, NGAL, and RBP Measurement in Early Diabetic Nephropathy Diagnosis

This study aimed to validate latex immunoturbidimetric assays for cystatin C (Cys-C), neutrophil gelatinase-associated lipocalin (NGAL), and retinol-binding protein (RBP), and to evaluate their combined clinical value for the early diagnosis of diabetic nephropathy (DN). 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 Mindray BS-2800 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 serum Cys-C, NGAL, and RBP-S provided a diagnostic sensitivity of 89.0%, specificity of 92.0%, and an AUC of 0.935. The corresponding urinary panel achieved a sensitivity of 82.0%, specificity of 88.0%, and an AUC of 0.876. In conclusion, the assays can be considered reliable. The combined detection of Cys-C, NGAL, and RBP in serum offers the highest diagnostic value for early DN, while urine analysis provides complementary information about tubular injury.