Background: Myalgic encephalomyelitis and chronic fatigue syndrome have case definitions with varying criteria, but almost all criteria require an individual to have a substantial reduction in activity level.
Unfortunately, a consensus has not been reached regarding what constitutes substantial reductions.
One measure that has been used to measure substantial reduction is the Medical Outcomes Study Short-Form-36 Health Survey (SF-36). 
Purpose: The current study examined the relationship between the SF-36, a measure of current functioning, and a self-report measure of the percent reduction in hours spent on activities.
Results: Findings indicated that select subscales of the SF-36 accurately measure significant reductions in functioning.
Further, this measure significantly differentiates patients from controls.
Conclusion: Determining what constitutes a significant reduction in activity is difficult because it is subjective to the individual.
However, certain subscales of the SF-36 could provide a uniform way to accurately measure and define substantial reductions in functioning.
Assessing current functioning as a measure of significant reduction in activity level, by Taylor Thorpe, Stephanie McManimen, Kristen Gleason, Jamie Stoothoff, Julia L. Newton, Elin Bolle Strand & Leonard A. Jason in Fatigue: Biomedicine, Health & Behavior [Published online: 19 Jul 2016]