Minimal important difference and patient acceptable symptom state for pain, Constant-Murley score and Simple Shoulder Test in patients with subacromial pain syndrome

BACKGROUND: The results of clinical trials should be assessed for both statistical significance and importance of observed effects to patients. Minimal important difference (MID) is a threshold denoting a difference that is important to patients. Patient acceptable symptom state (PASS) is a threshold above which patients feel well. OBJECTIVE: To determine MID and PASS for common outcome instruments in patients with subacromial pain syndrome (SAPS). METHODS: We used data from the FIMPACT trial, a randomised controlled trial of treatment for SAPS that included 193 patients. The outcomes were shoulder pain at rest and on arm activity, both measured with the 0–100 mm visual analogue scale (VAS), the Constant-Murley score (CS), and the Simple Shoulder Test (SST). The transition question was a five-point global rating of change. We used three anchor-based methods to determine the MID for improvement: the receiver operating characteristic (ROC) curve, the mean difference of change and the mean change methods. For the PASS, we used the ROC and 75th percentile methods and calculated estimates using two different anchor question thresholds. RESULTS: Different MID methods yielded different estimates. The ROC method yielded the smallest estimates for MID: 20 mm for shoulder pain on arm activity, 10 points for CS and 1.5 points for SST, with good to excellent discrimination (areas under curve (AUCs) from 0.86 to 0.94). We could not establish a reliable MID for pain at rest. The PASS estimates were consistent between methods. The ROC method PASS thresholds using a conservative anchor question threshold were 2 mm for pain at rest, 9 mm for pain on activity, 80 points for CS and 11 points for SST, with AUCs from 0.74 to 0.83. CONCLUSION: We recommend the smallest estimate from different methods as the MID, because it is very unlikely that changes smaller than the smallest MID estimate are important to patients: 20 mm for pain VAS on arm activity, 10 points for CS and 1.5 points for SST. We recommend PASS estimates of 9 mm for pain on arm activity, 80 points for CS, and 11 points for SST. TRIAL REGISTRATION: ClinicalTrials.gov NCT00428870 (first registered January 29, 2007). SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12874-021-01241-w.