Caroline B. Terwee, PhD
Department of Epidemiology and Data Science, Amsterdam Public Health research institute, Amsterdam UMC
Interpretation of change scores of patient-reported outcome measures (PROMs) is important to facilitate their use in research and clinical practice. There are several ways to interpret PROM change scores. One possible threshold is the minimal important change (MIC) estimate, which refers to the smallest change in score that patients consider important. There is a lot of confusion about the concept of MIC, which hampers and may even bias the interpretation of PROM change scores in research and clinical practice. In a recent paper that won the ISQOOL 2021 Quality of Life Research Outstanding Article of the Year award, we aimed to clarify the concept of MIC and how to use it.
We define the MIC as a threshold for a minimal within-person change over time above which patients perceive themselves as importantly changed. Assuming that all patients have their individual thresholds of what they consider a minimal important change, the MIC can be conceptualized as the mean of these individual thresholds. This definition of MIC is made up of three important elements: First, it refers to a threshold for a minimal change above which patients perceive themselves as changed (improved or deteriorated). Second, it refers to a change that is considered important to patients. And third, it refers to a within-patient change over time.
These three elements do not only define what the MIC is but also clarifies what the MIC is not. The MIC does not refer to thresholds for changes that are considered more than minimal. The MIC is also not a minimal detectable change (which refers to the smallest change in score that can be detected statistically with some degree of certainty). And finally, the MIC is not a difference between (groups of) patients.
A MIC value can be used for different purposes. Some use the MIC value as a threshold to determine the number of responders (i.e., patients who have a change at least as large as the MIC) in groups of patients who receive certain treatments (in studies or in clinical practice). This can be helpful to inform future patients about the expected effects of treatments. However, it is necessary to acknowledge that not all patients may be classified correctly with this approach because the estimated MIC value refers to the average of a group of patients, and the threshold may not apply to the individual patient in the consultation room. In clinical practice, the MIC value should therefore be considered a probabilistic value, rather than a deterministic cut-point, to interpret change scores in light of the probability that an individual patient has experienced a meaningful change.
To increase the understanding of the concept of MIC and improve the field, we need to agree on a clear definition of the MIC and report MIC values that are based on this definition. Also, MIC studies should be reported more clearly, because important details of MIC studies are often lacking in publications. We recommend reporting at least the percentage of patients improved, correlation between the PROM change score and the external criterion (anchor), samples size on which MIC value was based, and details on how the MIC value was estimated.
Finally, it is important to note that there is no perfect MIC method. We consider the predictive modelling method the most appropriate method at this time but the MIC methodology is still under study. In the full paper, linked here, we provided practical guidance for estimating methodologically sound MIC values.
This newsletter editorial represents the views of the author and do not necessarily reflect the views of ISOQOL.
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