BACKGROUND: An ideal test to evaluate the inflammatory burden in ulcerative colitis is still an unmet need. Fecal calprotectin (FCP) and C-reactive protein (CRP) have significant limitations. Plasma calprotectin (PC) seems to be promising in inflammatory diseases, but its value in IBD is still to be determined. Our aim was to assess whether PC correlates with inflammatory activity in UC. METHODS: Prospective single center cohort study. Consecutive patients previously diagnosed with UC undergoing endoscopy were included (June 2021-September 2022). Demographic, clinical, analytical (CRP, PC and FCP), endoscopic and histologic data was collected at the time of colonoscopy. PC was assessed with Gentian Calprotectin Immunoassay and, in a subgroup of patients, also with QUANTA Flash Circulating Calprotectin from INOVA. RESULTS: Inclusion of 98 patients (60.2% male) with a median age 49 (38-61) years. The extent of colitis was distal in 12 (12.2%), left-sided in 49 (50%), and extensive in 37 (37.8%). Mesalazine was taken by 65 (66.3%) patients, with biologic monotherapy used in 24 (24.5%) and combination therapy in 6 (6.1%). Clinical, endoscopic and histological remission were detected, in 56 (57.1%), 48 (49%) and in 55 (56.1%) patients, respectively. Comparing MES 0/1 vs MES 2/3, a statistically significant difference was found with PC, CRP and FCP. Concerning endoscopic (MES=1) and histological (GS<
2) remission, FCP was the only biomarker able to detect these outcomes. PC (Gentian) and PCi (INOVA) were highly correlated with CRP. CONCLUSION: PC has low value in distinguishing patients in remission from patients with endoscopic or histologic activity in UC. This essential role must continue be played by FCP.