Objectives: The 4-item Morisky Medication Adherence Scale (MMAS-4) and the more recent 8-item version (MMAS-8) have been both validated, and their concurrent validity has been assessed among hypertensive patients, but the extent to which the two scales can be compared against one another has not been determined. The current analysis assessed whether adherence scores obtained with the two scales on different patients can be compared or integrated across studies. Methods: Data were taken from the 2011 and 2012 US National Health and Wellness Survey (NHWS). The NHWS is a large cross-sectional survey representative of the total adult population in several major markets; current analyses were limited to the US (n-75,000/year). Respondents self-reported physician diagnosis of various health conditions, including 9,633 who reported a diagnosis of IBS. Adherence was measured with MMAS-4 in 2011 and MMAS-8 in 2012. The two adherence scales were evaluated by comparing the frequency distributions of the MMAS scores in the two scales, Cronbach's alpha and inter-item correlations, and the creation of a new 4-item scale including the questions in MMAS-8 that best matched the questions in MMAS-4. Results: In IBS patients, both MMAS-4 and -8 scores are Poisson-like distributed, with median at zero (high adherence). Chronbach's alpha was 0.64 for MMAS-4 and 0.70 for MMAS-8, while average item-test correlations were 0.70 and 0.59, respectively. The reduced 4-item scale created out of MMAS-8 is also Poissonlike distributed, Cronbach's alpha was 0.67 and the average item-test correlation was 0.71. Conclusions: Data obtained with the two MMAS show similar qualitative and quantitative characteristics, suggesting that it may be appropriate to integrate data sources using the two different versions, particularly when the responses to the subset of 4 MMAS-8 items are available. Future research should confirm that the scales can be integrated in different therapeutic areas.
Pedersini, R., Isherwood, G., & Vietri, J. (2013). Harmonizing Measurement of Adherence Across the 4-Item and 8-Item Morisky Medication Adherence Scale Using Cross-Sectional Data from Patients Treated for Irritable Bowel Syndrome. Value in Health, 16(7), A604. https://doi.org/10.1016/j.jval.2013.08.1719