Multidimensional Fatigue Inventory (MFI-20)

General Fatigue captures overall tiredness and lack of energy (subjective sensation: 'I am tired,' 'I am exhausted'). Physical Fatigue specifically measures bodily weakness and reduced capacity (physical sensation: 'I feel weak,' 'I get tired very quickly'). Both can be elevated in cancer fatigue, but they may dissociate in specific conditions. High general + low physical might suggest mood-related tiredness; high physical + low general might reflect unrecognized physical limitation.

Reduced Activity measures behavioral capacity and engagement ('I do not have energy for activities'). Reduced Motivation measures psychological drive and will ('I lack desire'). A patient with high motivation despite low activity capacity (wants to engage but cannot due to physical limitation) presents differently from low motivation with high activity capacity (can engage but lacks will, suggesting depression). Measuring both enables identification of paradoxical or discordant fatigue patterns.

MFI-20 was developed in cancer populations but has been extensively validated in chronic fatigue syndrome, ME/CFS, heart failure, and COPD. The five-dimensional model is disease-agnostic and conceptually relevant across conditions. Use in non-cancer populations is supported by published validation literature; however, normative data, cutoff thresholds, and responsiveness are condition-specific. Consult disease-specific validation studies before interpreting MFI-20 results in non-cancer populations.

Per-subscale clinically meaningful change: typically ≥5 points on the 4–20 scale. Total MFI meaningful change: ≥15–20 points on the 20–100 scale, depending on baseline severity and intervention type. Effect-size benchmarks (Cohen d, small ~0.2, medium ~0.5, large ~0.8) should supplement raw score interpretation. Anchor-based minimal clinically important differences specific to cancer fatigue or relevant chronic illness are preferable when available.