Minimum Clinically Important Difference
Minimum Clincially Important Difference (MCID) was originally defined as “the smallest difference in score in the domain of interest which patients perceive as beneficial and which would mandate, in the absence of troublesome side effects and excessive cost, a change in the patient’s management” (Jaeschke 1989), subsequently shortened to “the smallest change that is important to patients” (Stratford 1998). The first version attempts to divorce the change in symptoms which are the subject of interest from the costs, in terms of finance, side effects or inconvenience and is a very pure form of the concept – trying to determine what is the minimum change in a scale which the average patient would report as an improvement (or a deterioration), independent of other concerns. The second version, though much simpler, immediately begs the question – what is “important” to the average patient?
It turns out that there is no current consensus answer to how to define “important” and all attempts to measure the MCID are forced to choose some other marker of “importance to the patient” as the criterion for their particular estimate. In most studies an ‘anchor’ question or scale is used which defines symptom change in ordinary language. A simple example of such an anchor scale would be:
Do you consider your symptoms to be?
We may then decide that MCID for our scoring tool is that change which corresponds to the difference in score from before to after treatment in patients who report themselves “slightly better”. Note that there is no absolute reason for these things to be symmetrical – the average difference in change scores for patients who consider themselves slightly worse is not necessarily just a change of the same magnitude in the opposite direction as that for those who are slightly better. Furthermore in some contexts, where the aim of treatment, as in CTS, is to achieve “much better” there may be grounds for setting the target as the change in score corresponding to “much better’ rather than “slightly better”, though in such a case the term MCID becomes something of a misnomer as it is not really “minimal”. Changing the wording of the anchor scale may give entirely different results, for example one study of MCID for the SSS/FSS (Kim 2012) used the following anchor:
“Overall, how pleased are you with the outcome of surgery?
Not so pleased
MCID was then estimated as the score change which best differentiated the first two groups of patients from the last two – the answers being -1.14 for the SSS and -0.74 for the FSS
Another attempt to determine MCID for the SSS (Ozyurekoglu 2006) used a behavioural anchor – looking for the smallest improvement in SSS which corresponded to patients reaching a decision that no further treatment was required, as opposed to the group of patients who chose to have further interventions. By this method the MCID for SSS was estimated at -1.04
The anchor scale used in Canterbury is described in detail HERE
A very useful definition of “important’ can be obtained by asking a single Yes/No question after an intervention as follows:
If you were in the same position again and you knew that the outcome of treatment would be exactly the same as it has been this time, would you go ahead with the treatment?
This has the advantage of direct relevance to future decision making for similar patients. Variations of this include “Would you recommend this treatment to a friend or family member” and, in the case of a bilateral condition like CTS, “Would you have the other hand done?”
In the absence of an suitable anchor for comparison some authors have adopted a purely mathematical definition of MCID – such as half a standard deviation of the scores in the initial population.
Revision date - 23rd August 2015