Cysteine protease inhibitor C (Cystatin C, Cys-C), also known as cystatin C, widely present in nucleated cells and body fluids of various tissues, is a low molecular weight, basic non-glycated protein with a molecular weight of 13.3KD, composed of 122 amino acid residues, can be produced by all nucleated cells in the body, and the production rate is constant. The highly sensitive Cys-C paired antibody can be used for the evaluation of early damage to renal function in diabetic nephropathy, the early diagnosis of hypertensive renal function damage, the evaluation of renal function recovery in kidney transplant patients, the monitoring of renal function changes in hemodialysis patients, the evaluation of renal function in the elderly, the diagnosis of pediatric nephropathy, and the monitoring of renal function in tumor chemotherapy.