Background:Previous studies have proved that neutrophil gelatinase-associated lipocalin(NGAL)plays an important role in the progression of Crohn's disease(CD),and may serve as a potential biomarker for disease activity prediction,severity assessment,treatment response evaluation and prognosis monitoring.However,the diagnostic value of NGAL in perianal fistulizing Crohn's disease(pfCD)is still unclear.Aims:To investigate the serum level of NGAL and its diagnostic value in patients with active pfCD.Methods:A total of 66 patients diagnosed as pfCD from July 2021 to June 2023 at Longhua Hospital,Shanghai University of Traditional Chinese Medicine were enrolled,including 36 active pfCD patients and 30 inactive pfCD patients.The disease activity and perianal fistula activity were assessed by Crohn's disease activity index(CDAI)and perianal disease activity index(PDAI),respectively.Serum NGAL,fecal calprotectin(FC),C-reactive protein(CRP),erythrocyte sedimentation rate(ESR),as well as CDAI score and PDAI score were compared between the active and inactive pfCD patients,and the correlations of NGAL with the other parameters in active pfCD patients were analyzed.ROC curve was drawn to evaluate the values of serum NGAL,FC,CRP and ESR for diagnosis of active pfCD.Results:The serum NGAL,FC,CRP,ESR,CDAI score and PDAI score in active pfCD patients were significantly higher than those in inactive pfCD patients(all P＜0.001).NGAL was positively correlated with FC(r=0.64,P＜0.001),CRP(r=0.55,P＜0.001),ESR(r=0.53,P＜0.001),CDAI score(r=0.59,P＜0.001)and PDAI score(r=0.54,P＜0.001)in active pfCD patients.The optimal cut-off values of NGAL,FC,CRP and ESR were 220.5 μg/L,146.0 μg/g,7.9 mg/L and 23.5 mm/h,respectively,for the diagnosis of active pfCD,and the area under the curve were 0.922(95％CI:0.850-0.995),0.888(95％CI:0.806-0.970),0.853(95％CI:0.763-0.944)and 0.830(95％CI:0.731-0.930),respectively.Conclusions:Serum NGAL level is associated with the disease activity of pfCD,and can be used as a non-invasive biomarker for the clinical diagnosis of active pfCD.