Minimal clinically important difference, clinical perspective: an opinion.

The Minimal Clinically Important Difference has become a key feature for both the validation of clinical tools and for the assessment of clinical studies. Several methods have been developed to establish what a Minimal Clinically Important Difference is. The primary purpose of the Minimal Clinically Important Difference, however, is to provide a measure of relevance for a statistically applied measure. It does not, despite its name, necessarily relate to the "Clinical" condition. In this context, human beings are capable of extremely fine grades of discrimination of very subtle differences, when they care about the measures. When they do not care about the measure, large differences may be irrelevant. The flavors of wines or the tone qualities of musical instruments are readily recognizable examples. The importance of an outcome, for a clinician caring for an individual patient, therefore, will be highly patient specific. The Minimal Clinically Important Difference has great utility in assessing tools for clinical investigation. It has limitations in assisting the clinician. The subtleties that may be meaningful to individuals are often lost in discrimination tests in large populations where not all have the same interests. In addition, readily applicable tests, for example, discriminating degrees of salty water, for which Minimal Clinically Important Difference can be readily defined, are often tests that have no interest for the majority of the population tested. This leads to several paradoxes. Readily defined Minimal Clinically Important Differences are likely to be defined for parameters that are of little interest to a large number of persons. Conversely, parameters that are of great interest to selected individuals, that could be discerned by them with great subtly are likely to be poorly generalizable. Without doubt, defining a Minimal Clinically Important Difference will remain a key goal in the validation and application of tools for clinical investigations. The limits of the concept, particularly as it relates to issues of importance to patients, however, needs to be recognized.