TYPE: Abstract TOPIC: Lung Cancer PURPOSE: Procalcitonin (PCT) is a peptide released in response to an inflammatory stimulus used as a biomarker in bacterial infections. Currently, it is unknown if immunotherapy (IO) raises PCT. Anti-PD(L)1 antibodies are drugs that trigger the activation of the T-lymphocytes on malignant cells, releasing pro-inflammatory cytokines. This study aims to describe PCT fluctuations after IO and evaluate its role as a biomarker of infection. METHODS: Lung cancer patients treated with IO (anti PD(L)-1 and CTLA4) were recruited. Before administering IO, PCT was quantified by chemiluminescent immunoassay and 21 days after the first and second doses. Patients with suspected or confirmed infections were excluded. Longitudinal data were analyzed using a linear mixed-effect model. RESULTS: Twenty-six patients completed PCT determinations. Most patients were former tobacco smokers (69.2%), with lung adenocarcinoma diagnosis (76.9%) and advanced disease (90.6%). The most frequent treatment was anti-PDL1 alone (11.5%) or combined with chemotherapy (88.5%). Compared with baseline PCT levels, there was no significant fluctuation of serum concentrations after starting IO at 21 days (p=0.061) and 42 days (p=0.077) (Fig. 1A). There was a significant effect between tobacco consumption (p = 0.009), biomass exposure (p = 0.01), and PCT level; however, the correlation did not modify the PCT direction (Fig. 2 B). CONCLUSIONS: PCT did not show a significant elevation after IO, making it a helpful biomarker to detect infectious diseases and differentiate them from an immuno-associated effect like pneumonitis. CLINICAL IMPLICATIONS: PCT is a useful early marker of sepsis in cancer patients and is commonly used as a guide for managing antibiotic therapy. DISCLOSURE: M Arroyo-Hernández is speaker for Astra Zeneca and Bristol receiving 500 dls per meeting L Cabrera-Miranda and M Ramos-Ramírez are speakers for Astra Zeneca, Roche, Ely, Lilly and Boheringer Ingelheim receiving 500 dls per meeting AF Cardona, O Arrieta ar KEYWORD: cancer/biomarker