Paediatric Bronchiectasis (PB) diagnosis often stems from expert radiological opinion alone. Recent ERS guidelines suggest using broncho-arterial ratios (BAR) with a ?0.8 cut-off rather than 1.0 used in adults as it may underemphasise PB severity. It is unclear whether inner (IAD) or outer airway diameters (OAD) should be used.

Aims: Compare adult vs paediatric BAR cut-off criteria in PB diagnosis. Compare IAD and OAD in BAR calculations.

Methods: Retrospective analysis of 64 children with labelled PB from electronic records (2009-2020) at a tertiary hospital. 96 HRCTs were reviewed. IAD and OAD BAR measurements were taken. The largest of multiple measurements was used. BAR and wall thickening ratio (WTR) were calculated.

Results: The mean age at HRCT diagnosis was 5 years (SD3.35) (30M, 34F). 24(48%) additional patients were labelled with PB using inner BAR?0.8 compared to BAR ?1. 159 IAD and OAD measurements were taken after excluding patients with repeat HRCTs. 142/159 (89%) had an outer BAR?0.8 and 83/159 (52%) had an inner BAR?0.8 (p<0.0001). Using IAD compared to OAD to measure BAR, PB diagnosis (BAR?0.8) was reduced by 37%. There was a significant difference in mean WTR between patients with BAR?1(0.627mm (SD0.237)) and BAR<1(0.455(SD0.09612)) (p=0.001), but no significant difference between patients with BAR?0.8 and BAR<0.8 (p=0.444). There was a significant correlation between outer BAR and WTR (p=0.006).

Conclusion: Using BAR?0.8 compared to ?1.0 increased the diagnostic label of PB and needs further evaluation regarding sensitivity vs specificity and clinical outcome. IADs may underemphasize PB extent due to mucus attachment to airway walls causing mucosa folding. WTR may provide an alternative marker.