Minimal important differences of the SRS-22 Patient Questionnaire following surgical treatment of idiopathic scoliosis

The responsiveness of an instrument measuring health-related quality of life is an important indication of its construct validity. The SRS-22 Patient Questionnaire has become the most widely used patient-reported outcome instrument in the clinical evaluation of patients with idiopathic scoliosis. The responsiveness of the SRS-22 following surgical treatment in patients with idiopathic scoliosis has not been fully assessed. The aim of this study is to evaluate this factor by calculating the minimal important differences (MIDs) of the SRS-22 Questionnaire. The study included 91 patients with idiopathic scoliosis (77 females and 14 males), who underwent surgical treatment; mean age at the time of surgery was 18.1 years. Patients completed the SRS-22 questionnaire before surgery and at a follow-up visit (mean follow-up, 45.6 months). At follow-up, patients rated their overall situation as related to before surgery with a four-point Likert scale: 1-Worse, 2-Same, 3-Better, 4-Much Better. This evaluation represented the global perceived effect (GPE) and served as the anchor criterion for calculating the MID. MIDs were calculated using two approaches. The anchor-based MID (MID-A) was defined as the mean preoperative/follow-up difference in SRS-22 scores in the group of patients who stated they were much better than before surgery (GPE = 4). Using the same anchor criterion, the optimal cut-off value able to identify patients that had clearly improved was determined on a receiver operating characteristic (ROC) curve. In addition, the distribution-based MID (MID-D) was calculated by the standard error of measurement method. The MID-As found for the different subscales and the sum score were: pain 0.6, function 0.3, image 1.3, mental health 0.3, average sum score 0.6, and raw sum score 13.1. The cut-off values on the ROC curve were: pain 0.2, function 0.0, image 1.6, mental health 0.4, average sum score 0.4, and raw sum score 10. The MID-Ds were: pain 0.6, function 0.8, image 0.5, mental health 0.4, average sum score 0.5, and raw sum score 6.8. As was expected, the MID values differed according to the calculation method used. In light of the fact that the MID-As for the function and mental health subscales are below the measurement error of the instrument, it seems preferable to use the MID-D values for determining subscale changes. If the purpose is to analyze sum score changes (either the raw or average values), the MID-A is preferable because it includes the patient's evaluation of the results of surgery.