Repeatability and replicability of the Respiratory Symptom Experience Scale (RSES): internal and external validation of smoking-status differences in respiratory symptoms

Respiratory symptoms are common among smokers without COPD, yet symptom instruments are often optimized for established disease and may under-capture early changes relevant to cessation and tobacco harm reduction. We conducted a repeatability and replicability study of the Respiratory Symptom Experience Scale (RSES) in a rigorously phenotyped cohort to confirm both measurement stability and reproducible discrimination by smoking status. Healthy adults were recruited at a UK hospital and classified as current, former, or never smokers using self-report, exhaled CO, and a comprehensive panel of biomarkers of exposure. RSES was administered at Visit 1 and repeated after 7 ± 3 days at Visit 2 to assess test–retest repeatability. Known-group validity (discriminability by smoking status) was evaluated by comparing RSES scores across biochemically verified groups using non-parametric tests, with ANCOVA for exploratory covariate adjustment. True replicability across independent datasets was not formally tested in the present study. Of 180 enrolled participants, 170 were included (55 current smokers, 60 former smokers, 55 never smokers) after excluding clinical ineligibility and biomarker-inconsistent smoking status. RSES demonstrated high repeatability (Pearson r = 0.87; Spearman ρ = 0.85) with minimal bias (− 0.07) and good agreement. RSES also showed consistent known-group discrimination by smoking status, differing between current and never smokers ( p < 0.0001) and across all three groups ( p < 0.0001). Current smokers reported higher scores than never (median 2.0 vs. 1.0) and former smokers (2.0 vs. 1.2), whereas former and never smokers did not differ ( p = 0.686). Covariate adjustment did not materially alter results. RSES shows strong repeatability and consistent known-group discrimination by smoking status, supporting its use as a patient-relevant endpoint for cessation, and harm-reduction trials, as well as for clinical respiratory and regulatory research.