S135 The added value of nasal potential difference measurement when first-line cystic fibrosis (CF) Investigations are non-diagnostic

Introduction The diagnosis of CF can be challenging when the sweat chloride (Cl−) value is non-diagnostic ( Methods Retrospective study of medical records of all patients referred to our DCFD service (2009–2017). Final diagnoses were reached by consensus opinion contemporaneously on review of all available data. Results 172 patients were reviewed: 129 (75.0%) ≥16 years; 78 (45.3%) male; median (range) age 28.9 (2.9–76.5) years. 136 (79.1%) had a history of chronic lower respiratory tract symptoms (of which 58.2% had bronchiectasis)), 33% chronic sinusitis and 6% pancreatitis. Of male patients≥16 years (n=63), 16 (25.4%) had confirmed congenital bilateral absence of the vas deferens. The median (range) sweat Cl− of the whole group (n=160) was 42.3 (2.0–109) mmol/L; 12 patients produced insufficient sweat. For first-line genotyping (up to 50 mutations) 78 (45.3%) patients had mutations/variants identified (n=24, compound heterozygote – nil diagnostic); extended CFTR analysis increased the diagnostic yield by 3 patients. Using nPD the diagnosis was clarified for 145 (84.3%) patients, by excluding CF (n=101; 58.7%) or confirming CF/CFTR-related disorder (RD) (n=44; 25.6%). The others remained diagnostic uncertainties (n=10), were technical failures (n=7) or were associated with a possible sodium channel defect (n=10). Sodium channel mutation analysis is currently underway. Adults were more likely to have CF/CFTR-RD confirmed than children (30% v 9%). 18 (10.5%) patients had their long-standing CF diagnosis reversed. Conclusions Our DCFD service – with the inclusion of nPD – improves the diagnostic yield in 84.3% of cases with equivocal first-line CF investigations. With our recently introduced extended (CFTR and non-CFTR) genomic service, we expect this will increase further. Our service provides important diagnostic clarity to patients, caregivers and healthcare providers.