When Minimal Detectable Change Exceeds a Diagnostic Test–Based Threshold Change Value for an Outcome Measure: Resolving the Conflict

Assessing patient progress is an integral part of physical therapist practice. In an attempt to assist clinical decision making regarding a patient's change status, researchers have offered study-based threshold change values. Often researchers have provided reliability and diagnostic test–based estimates of threshold change values obtained from the same patient sample. A potential dilemma occurs when the reliability (ie, the minimal detectable change [MDC])–based threshold change value exceeds the diagnostic test–based threshold value. How can a change be detected if the threshold change value falls within the limits of error? In this situation, researchers have recommended using the larger MDC threshold change value. In this perspective article, we describe the interpretation of the threshold values provided by each of these estimation methods and consider which one offers information that is more meaningful to the challenge faced by physical therapists when making decisions concerning the change status of patients. The context for our discussion is a clinical vignette that depicts the dilemma outlined above. We conclude this perspective with suggestions for researchers concerning essential information to include when reporting threshold estimates obtained from reliability–based and diagnostic test–based studies of outcome measures.