Doumat, Georgina

Abstract RATIONALE: Absolute Blood Eosinophil Counts (BEC) in bronchiectasis patients offer insights into inflammatory patterns and potential treatments. In non-cystic fibrosis bronchiectasis (NCFB), BEC≥300 cells/μL characterizes Eosinophilic Bronchiectasis (EB), a type-2 inflammation endotype estimated in 20% of patients. Guidance on BEC assessment strategy (i.e., number, frequency and timing of BEC≥300 defining this endotype) is unclear. We hypothesize that EB incidence varies by BEC≥300 assessment strategy. We also investigate eosinophilic inflammation in CF for comparison. Objectives: Assess BEC≥300 incidence in NCFB and CF cohorts. METHODS: From January 2023 to October 2024, BEC were measured in 9,953 stable bronchiectasis patients across Mayo Clinic, including 9,806 with NCFB (J47.9; 66,780 CBCs) and 147 with CF (E84.0; 1,405 CBCs). Incidence of BEC≥300 was evaluated in relation to CBC frequency and timing (i.e., BEC≥300 on first, second, last, any, any two, first two simultaneously) and the number of CBCs per patient. RESULTS: In the NCFB cohort, median age was 74 years (IQR:66-82), with 59.5% female. Median CBCs/patient was 3 (IQR:1-7). Incidence of BEC≥300 on the first CBC was 19.8%. Among patients with ≥2 CBCs (n=7,075), incidence was 20.1% on the first, 19.8% on the second, and 20.0% on the last. This incidence rose to 36.3% when considering any CBC/patient. BEC≥300 on at least two CBCs was 29.3%, decreasing to 11.8% when considering consecutive BEC≥300 on both first and second CBCs. The incidence of BEC≥300 increased linearly with the number of CBCs obtained: 24.6% for those with 1-3 CBCs, up to 64.6% for >12 CBCs. In the CF cohort, median age was 36 years (IQR:27-52), with 55.1% female. Median CBCs/patient was 4 (IQR:2-8). BEC≥300 was present in 22.9% of first CBCs, rising to 36.3% when considering any CBC and 29.3% when present on at least two CBCs. Similar to the NCFB cohort, incidence increased with the number of CBCs: 26.5% for 1-3, up to 77.3% for >12. CONCLUSIONS: The incidence of BEC≥300 in our NCFB cohort remained steady at 20% at any specific measurement point, consistent with prospective literature. However, nearly half of those with BEC≥300 on the first or second CBC did not meet this threshold on both. Incidence significantly increased when any CBC was considered, and more CBCs were available. Similar observations were made in the CF cohort. Clearer BEC assessment strategies are needed to define clinically significant eosinophilic inflammation in bronchiectasis, to better identify patients who may most benefit from type 2 inflammation-targeted treatments.