Objectives:Fecal calprotectin (FC) is a well-integrated parameter in the monitoring of adolescent patients with inflammatory bowel disease (IBD). However, measurement of FC is limited by day-To-day-variation and by the feces consistency. Furthermore, adolescents are often noncompliant to deliver fecal sampling leading to suboptimal monitoring. Consequently, we see the need of a substitute biomarker whenever measurement of FC fails and aimed to investigate serum calprotectin (SC) in adolescents with IBD.Methods:In cross sectional data from 19 ulcerative colitis (UC) patients <18 years old, a Spearman correlation was used to analyze the correlation between SC, FC, C-reactive protein (CRP) and endoscopic and symptom scores. In longitudinal data collected from 20 UC and Crohn disease (CD) patients (10-17 years old), Mixed Effect Models (MEM) were used to analyze the association between SC, FC, CRP, and symptom scores.Results:We found positive correlations between SC (19 samples) and the endoscopic score, symptom score, and CRP (r=0.56, P=0.01; r=0.64, P=0.003; r=0.97, P<0.0001). We found no significant correlation between SC and FC. In 27 samples from UC patients, the association of SC with FC and CRP were positive and significant (P=0.004, estimate=0.32; P=0.0001, estimate=0.002). The association between SC and symptom score was insignificant. In 49 samples from CD patients, the association between SC and CRP was significant (P=0.02, estimate=0.002) whereas associations between SC and FC and symptom score were insignificant.Conclusions:In the current pilot study, we found a correlation between SC and the endoscopically assessed inflammation in UC. SC may have the potential to improve disease monitoring of adolescent patients.