Lung Clearance Index in adult patients with atypical Cystic Fibrosis and normal FEV1

<bold>Background:</bold> CF manifests in childhood with respiratory and/or digestive symptoms. However, due to the diversity of mutations, various phenotypes are observed ranging from the most common manifestations to more atypical or even milder manifestations. Occasionally, patients with atypical CF can have normal spirometry, so LCI may be useful to identify early lung damage. <bold>Objetives:</bold> To analyse whether adult patients with atypical CF present abnormal values in LCI. To correlate LCI results with other respiratory and non-respiratory parameters of the disease. <bold>Methods:</bold> Prospective observational study from May to December 2023. Twenty-seven patients with atypical CF were included. All patients underwent spirometry, 6MWT, CFQ-R (Respiratory) questionary, blood test and LCI. LCI was performed using the Exhalyzer D machine and N2 as tracer gas. A minimum of 3 leak-free tests were required. Normal cutoff value of LCI was set at 7 according 3.3.1 software (ULN for LCI). <bold>Results (Table 1-2):</bold> All patients had an LCI≥7 at baseline. During the study period 2 patients suffered an exacerbation with a worsening in LCI from of 3.5; FEV₁ 1.5%; FVC 5%; FEF_25-75% 3%. There was no statistically significant correlation between baseline LCI and any of the respiratory and non-respiratory parameters analysed. <fig><object-id>erj;64/suppl_68/PA2348/F1</object-id><object-id>F1</object-id><object-id>F1</object-id><graphic></graphic></fig> <bold>Conclusions:</bold> In patients with atypical CF, the LCI can be more sensitive to precocious lung ventilation disorders than FEV₁ and FEF_25-75%.