Minimally important difference of the Treatment Satisfaction with Medicines Questionnaire (SATMED-Q)

BACKGROUND: A previous study has documented the reliability and validity of the Treatment Satisfaction with Medicines Questionnaire (SATMED-Q) in exploring patient satisfaction with medicines for chronic health conditions in routine medical practice, but the minimally important difference (MID) of this tool is as yet unknown. The objective of this research was to estimate the MID for the SATMED-Q total score and six constituent domains. METHODS: The sample of patients (456 subjects, mean age 59 years, 53% male) used for testing psychometric properties was also used to assess MID. Item #14 of the Treatment Satisfaction Questionnaire for Medication (TSQM) was used as an anchor reference since it directly explores satisfaction with medicine on a 7-point ordinal scale (from extremely satisfied to extremely dissatisfied, with a neutral category). Patients were classified into four categories according to responses to this item (extremely satisfied/dissatisfied, very satisfied/dissatisfied, satisfied/dissatisfied, neither satisfied nor dissatisfied (neutral), and calculations were made for the total score and each domain of the SATMED-Q using standardised scores. The mean absolute differences in total score (and domains) between the neutral category and the satisfied/dissatisfied category were considered to be the MID. Effect sizes (ES) were also computed. RESULTS: The MID for the total score was 13.4 (ES = 0.91), while the domain values ranged from 10.3 (medical care domain, ES = 0.43) to 20.6 (impact on daily living, ES = 0.85). Mean differences in satisfaction (as measured by the total SATMED-Q score and domain scores) using the levels of satisfaction established by item #14 were significantly different, with F values ranging from 12.2 to 88.8 (p < 0.001 in all cases). CONCLUSION: The SATMED-Q was demonstrated to be responsive to different levels of patient satisfaction with therapy in chronically ill subjects. The MID obtained was 13.4 points for the overall normalised scoring scale, and between 10.3 and 20.6 points for domains.