Background: The minimally important difference (MID) has been defined as the smallest improvement considered worthwhile by a patient. The MID has not been estimated for the Rhinoconjunctivitis Total Symptom Score (RTSS). Methods: In a prospective multicentre study, patients consulting for grass-pollen-induced allergic rhinitis (AR) recorded a 15-point global rating of change scale (GRCS) score and the Rhinoconjunctivitis Quality of Life Questionnaire (RQLQ) score on a weekly basis and the individual symptom scores comprising the RTSS on a daily basis over two consecutive weeks. The MID in the RTSS was determined with anchor-based methods (using the GRCS and the RQLQ) and a distribution-based method [based on the RTSS’ standard deviation (SD)]. Results: The study population comprised 806 patients (253 children, 250 adolescents and 303 adults). During the first week of the study, the mean +/- SD RTSSs for these age groups were 6.5 +/- 3.3, 6.8 +/- 3.4 and 7.0 +/- 3.4, respectively. For an improvement of 2 points in the GRCS or 0.5 points in the RQLQ score, the regression analysis yielded MIDs in the RTSS of 1.24 +/- 0.17 and 1.12 +/- 0.14 in children, 1.33 +/- 0.14 and 1.20 +/- 0.13 in adolescents and 1.13 +/- 0.14 and 0.89 +/- 0.12 in adults, respectively. When applying distribution-based methods, the MID ranged from 1.09 to 1.13 (based on 0.33 SDs of the first-week RTSS) and from 1.22 to 1.40 (based on 0.5 SDs of the difference in RTSSs between the first and second weeks). Conclusion: The MID in the RTSS was consistently estimated as 1.1-1.3 (and could conceivably be rounded to 1) in patients with grass-pollen-induced AR.
