Estimating minimal clinically important difference (MCID) for gastrointestinal symptoms in cystic fibrosis

Background: Minimal clinically important difference (MCID) is important to establish as a meaningful outcome in research when using patient reported outcome measures (PROMs). We determined the MCID using the distribution-based approach for three measurements used as part of the GALAXY study, which is an observational prospective study on gastrointestinal (GI) symptoms in cystic fibrosis (CF). Methods: Four hundred and two persons with cystic fibrosis (PwCF) participated in the GALAXY study, all with baseline values available for all questionnaires. Mean age was 20.9 years (2.1- 61.1) with 75 females and 94 males under the age of 18 (42.04 %) and 118 females and 115 males aged 18 or older (57.99 %). MCID was measured for Patient Assessment of Constipation Symptoms (PAC-SYM), Patient Assessment of Upper Gastrointestinal Symptoms (PAGI-SYM), Patient Assessment of Constipation-Quality of Life (PAC-QOL) and their subscales. Two distribution-based approaches, defined as multiplications of the standard deviation (SD) or standard error of the mean (SEM), were used to approximate the MCID. Results: The two distribution-based approaches for determining the MCID estimates produced comparable results in trends in MCIDs across the subscales and total scores. In general, MCID estimates of subscales for all three measurements were higher than their total score MCIDs. The one-half SD- and SEM-based MCID estimates for total scores of each questionnaire are as follows: PAC-SYM: 0.26 and 0.14; PAGI-SYM: 0.32 and 0.15; PAC-QOL: 0.27 and 0.18, respectively. Conclusion: This paper establishes initial MCIDs estimated by the distribution-based approach for the PAC-SYM, PAGI-SYM and PAC-QOL that can now be used to evaluate interventional studies that may impact gastrointestinal symptoms in PwCF.