Hypersensitivity Pneumonitis with and without Autoimmune Features: a comparative analysis at diagnosis

<bold>Introduction:</bold> Hypersensitivity Pneumonitis with Autoimmune Features (HPAF) remains a controversial poorly understood entity, being questionable whether it represents a simple finding or a distinct Hypersensitivity Pneumonitis (HP) clinical subgroup. <bold>Aim and objectives:</bold> to compare retrospectively diagnostic presentations of HPAF and HP without autoimmunity (HPc). <bold>Methods:</bold> All HPAF cases ever followed at a single central reference hospital in Portugal until October 2023 were included. A sample of HPc patients followed at the same center was randomly selected. All diagnosis were discussed at multidisciplinary team meeting. <bold>Results:</bold> HPAF (n=52) and HPc (n=51) groups were analysed. Both groups had similar demographics and smoking habits. Environmental exposure was similar identified in HAPF and HPc groups, respectively (avian: 81% vs 88%; p=0,296; mold: 27% vs 43%; p=0,084). At diagnosis, both groups had mild dyspnea (median mMRC=1; p=0.105). Median time from symptoms onset to diagnosis was 6 months [0.5-24] in HPAF group, significantly shorter than HPc group (12 months [7-24]; p= 0,008). Similar BAL lymphocytosis were documented (HPAF 24%[13-40], HPc 26%[13-48]; p=0.796). No significant differences at baseline pulmonary function tests or oxygen requirements were found. Regarding HRCT features, fibrotic HP (HPAF 71%, HPc 88%; p=0.031) and honeycombing pattern (HPAF 19%, HPc 41%; p= 0,015) were significantly higher in HPc than HPAF. <bold>Conclusions:</bold> HPAF patients exhibited a shorter symptomatic period and less fibrotic disease at presentation, suggesting a potentially better prognosis. The real impact of autoimmune features in HP needs further clarification to personalize patient management.