Extracellular traps are evident in Romanowsky‐stained smears of bronchoalveolar lavage from children with non‐cystic fibrosis bronchiectasis

Background and Objective The importance of extracellular traps (ETs) in chronic respiratory conditions is increasingly recognized but their role in paediatric bronchiectasis is poorly understood. The specialized techniques currently required to study ETs preclude routine clinical use. A simple and cost‐effective ETs detection method is needed to support diagnostic applications. We aimed to determine whether ETs could be detected using light microscopy‐based assessment of Romanowsky‐stained bronchoalveolar lavage (BAL) slides from children with bronchiectasis, and whether the ETs cellular origin could be determined. Methods Archived Romanowsky‐stained BAL slides from a cross‐sectional study of children with bronchiectasis were examined for ETs using light microscopy. The cellular origin of individual ETs was determined based on morphology and physical contact with surrounding cell(s). Results ETs were observed in 78.7% (70/89) of BAL slides with neutrophil (NETs), macrophage (METs), eosinophil (EETs) and lymphocyte (LETs) ETs observed in 32.6%, 51.7%, 4.5% and 9%, respectively. ETs of indeterminate cellular origin were present in 59.6% of slides. Identifiable and indeterminate ETs were co‐detected in 43.8% of slides. Conclusion BAL from children with bronchiectasis commonly contains multiple ET types that are detectable using Romanowsky‐stained slides. While specialist techniques remain necessary to determining the cellular origin of all ETs, screening of Romanowsky‐stained slides presents a cost‐effective method that is well‐suited to diagnostic settings. Our findings support further research to determine whether ETs can be used to define respiratory endotypes and to understand whether ETs‐specific therapies may be required to resolve airway inflammation among children with bronchiectasis. The burden of paediatric bronchiectasis is an ongoing global challenge. Extracellular traps (ETs) are a potentially important marker for defining paediatric bronchiectasis endotypes. We demonstrate Romanowsky‐stained BAL can be used as a simple and cost‐effective ETs detection method. Using this approach, we determined that ETs were present in BAL slides from 79% of 89 children with non‐cystic fibrosis bronchiectasis. Multiple ET types were observed, with ETs derived from macrophages (METs) and neutrophils (NETs) the most commonly detected types. See related editorial