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Introduction Chronic breathlessness represents a substantial and growing burden on patients and healthcare services, driving repeated referrals, hospitalisation, and multispecialty review. National guidance recommends spirometry as the first-line investigation; however, spirometry alone may be insufficient, potentially delaying diagnosis and increasing healthcare cost burden. While spirometry is frequently delivered in nurse-led settings, gas transfer measurement within physiologist-led services extends assessment beyond ventilatory mechanics and improves diagnostic yield. 1 This service evaluation examines whether early incorporation of gas transfer within a new local breathlessness pathway enables clinically useful phenotyping to improve triage, pathway efficiency, and patient care.Methods A retrospective service evaluation was conducted of adult patients assessed via a chronic breathlessness pathway. All patients underwent initial physiological assessment including spirometry and single-breath gas transfer. Patients were grouped by spirometry and gas transfer status. Continuous variables were summarised as median (IQR) and categorical variables as counts and percentages. Between-group comparisons used Mann–Whitney U tests for continuous variables and Fisher’s exact test for categorical variables. Clinical governance approval was obtained.Results Fifty-one patients completed both spirometry and gas transfer. Eight of 51 (15.7%; 95% CI 7.0–28.6) demonstrated normal spirometry with abnormal gas transfer. In this subgroup, TLCO z-scores ranged from −4.24 to −1.73, with six classified as mild and two as severe by ERS/ATS criteria. Patients with abnormal gas transfer had higher BMI (median 32.9 vs 29.9 kg·m −2; p=0.020) and dyspnoea scores (median 3 vs 2; p=0.073), with no difference in age (p=0.236). Patients with isolated gas transfer impairment were more frequently assigned actionable respiratory or pulmonary vascular diagnoses, while those with normal physiology were discharged without a specific diagnosis.Conclusions Early incorporation of gas transfer within a breathlessness pathway identifies clinically relevant abnormalities that would otherwise be missed by spirometry alone. These findings support a physiology-led, targeted approach to breathlessness assessment, using clinical phenotype to prioritise gas transfer and guide downstream investigation. Ongoing data collection will allow refinement of phenotypic thresholds and triage strategies.Reference Sylvester, et al. BMJ Open Respir Res. 2021.