The ROC analysis gave an AUC of 0.854 for serum Cat-S in diagnosing decreased renal function of DKD, and when the cut-off value of serum Cat-S was 974.14 pg/mL, the sensitivity and specificity of serum Cat-S in diagnosing DKD were 85% and 80% respectively. This evidence concerns the gene CAT and diabetic kidney disease.