Establishing how much improvement in lung function and distance walked is clinically important for adult patients with Pompe disease

Background and purpose Pompe disease is a rare, inheritable, progressive metabolic myopathy. This study aimed to estimate the minimal clinically important difference (MCID) for an improvement in forced vital capacity in the upright seated position (FVC up ) and the 6‐min walk test (6MWT) after a year of treatment with enzyme replacement therapy. Methods Data were obtained from two prospective follow‐up studies. Between‐group and within‐group MCIDs were estimated using anchor‐based methods. Additionally, a distribution‐based method was used to generate supportive evidence. As anchors, self‐reported change in health and in physical functioning, shortness of breath and a categorization of the Short‐Form 36 Physical Component Summary score were used. Anchor appropriateness was assessed using Spearman correlations (absolute values ≥0.29) and a sufficient number of observations in each category. Results In all, 102 patients had at least one FVC up or 6MWT measurement during enzyme replacement therapy. Based on the anchors assessed as appropriate, the between‐group MCID for an improvement in FVC up ranged from 2.47% to 4.83% points. For the 6MWT, it ranged from 0.35% to 7.47% points which is equivalent to a distance of 2.18–46.61 m and 1.97–42.13 m for, respectively, a man and a woman of age 50, height 1.75 m and weight 80 kg. The results of the distribution‐based method were within these ranges when applied to change in the outcome values. Conclusion The MCIDs for FVC up and 6MWT derived in this study can be used to interpret differences between and within groups of patients with Pompe disease in clinical trials and cohort studies.