Assessing health status over time: impact of recall period and anchor question on the minimal clinically important difference of copd health status tools

BACKGROUND: The Minimal Clinically Important Difference (MCID) assesses what change on a measurement tool can be considered minimal clinically relevant. Although the recall period can influence questionnaire scores, it is unclear if it influences the MCID. This study is the first to examine longitudinally the impact of the recall period of an anchor question and its design on the MCID of COPD health status tools using the COPD Assessment Test (CAT), Clinical COPD Questionnaire (CCQ) and the St. George’s Respiratory Questionnaire (SGRQ). METHODS: Moderate to very severe COPD patients without respiratory co-morbidities were recruited during 3-week Pulmonary Rehabilitation (PR). CAT, CCQ and SGRQ were completed at baseline, discharge, 3, 6, 9 and 12 months. A 15-point Global Rating of Change scale (GRC) was completed at each follow-up. A five-point GRC was used as second anchor at 12 months. Mean change scores of a subset of patients indicating a minimal improvement on each of the anchor questions were considered the MCID. The MCID estimates over different time periods were compared with one another by evaluating the degree of overlap of Confidence Intervals (CI) adjusted for dependency. RESULTS: In total 451 patients were included (57.9 ± 6.6 years, 65% male, 50/39/11% GOLD II/III/IV), of which 309 completed follow-up. Baseline health status scores were 20.2 ± 7.3 (CAT), 2.9 ± 1.2 (CCQ) and 50.7 ± 17.3 (SGRQ). MCID estimates for improvement ranged − 3.1 to − 1.4 for CAT, − 0.6 to − 0.3 for CCQ, and − 10.3 to − 7.6 for SGRQ. Absolute higher – though not significant – MCIDs were observed for CAT and CCQ directly after PR. Significantly absolute lower MCID estimates were observed for CAT (difference − 1.4: CI -2.3 to − 0.5) and CCQ (difference − 0.2: CI -0.3 to −0.1) using a five-point GRC. CONCLUSIONS: The recall period of a 15-point anchor question seemed to have limited impact on the MCID for improvement of CAT, CCQ and SGRQ during PR; although a 3-week MCID estimate directly after PR might lead to absolute higher values. However, the design of the anchor question was likely to influence the MCID of CAT and CCQ. TRIAL REGISTRATION: RIMTCORE trial #DRKS00004609 and #12107 (Ethik-Kommission der Bayerischen Landesärztekammer). ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1186/s12955-018-0950-7) contains supplementary material, which is available to authorized users.