Late Breaking Abstract - Single and multiple N2-washout tests for the diagnosis of bronchiolitis obliterans syndrome after allogeneic stem cell transplantation – a longitudinal study

<bold>Background:</bold> Bronchiolitis obliterans syndrome (BOS) is a severe pulmonary complication that arises after allogeneic stem cell transplantation (alloHSCT). The nitrogen single breath washout (N2SBW) and multiple breath washout (N2MBW) tests are employed to evaluate ventilation inhomogeneity across various levels of the airways. <bold>Methods:</bold> In this prospective, longitudinal, observational study, patients scheduled for alloHSCT were followed for two years. Comprehensive evaluations, including clinical, laboratory, radiological, and lung function tests, were performed. Mixed logistic regression analyses were used to compare patients who developed BOS with those who did not. <bold>Results:</bold> A total of 1440 visits from 166 patients were evaluated, with a mean age of 53 years (±14 years, 31% female). According to the NIH 2014 criteria and as assessed by the measurements at baseline before alloHSCT, the LCI 2.5, Sacin, and Scond z-scores from the N2MBW were similar between groups, but the SIII z-score from the N2SBW was significantly different (p=0.0028) in patients who developed BOS during the follow-up. The risk of developing BOS increased significantly with higher LCI 2.5 z-scores (OR: 1.0695, p=0.0214), Sacin z-scores (OR=1.1050, p=0.0021), and SIII z-scores (OR=1.3002, p<0.0001) during the two-years follow-up. The increase in risk associated with higher Scond z-scores was smaller and not statistically significant. <bold>Conclusions:</bold> Measuring exhaled N2 nitrogen may serve as a prognostic test to identify patients at risk for, or with incident, bronchiolitis obliterans syndrome following allogenic stem cell transplantation.