Assessing the relation between fatigue and depression in multiples sclerosis: the impact of measurement choice

Fatigue and depression are among the most prevalent and disabling symptoms in multiple sclerosis (MS) and are frequently reported to be strongly associated. This association is often interpreted as reflecting shared pathophysiology. However, widely used depression scales such as the Beck Depression Inventory-II (BDI-II) include somatic items that overlap with core features of MS and MS-related fatigue, potentially inflating observed relationships.

To examine whether the association between depression and fatigue in MS depends on the choice of depression measure, we compared the BDI-II with the Beck Depression Inventory-Fast Screen (BDI-FS), a subscale designed to minimize somatic confounding.

In a cohort of 86 people with MS, self-reported fatigue (MFIS, WEIMuS) and depressive symptoms (BDI-II, BDI-FS) were assessed. Fatigue scores were standardized to allow comparability across instruments. Pearson correlations were computed to quantify associations between depression and fatigue, and dependent correlations were compared.

Depressive symptoms assessed with the BDI-II showed strong positive correlations with fatigue (r = .60 -.66, all p <.001). In contrast, BDI-FS scores showed significantly weaker, moderate associations (all r ≤.43, p <.001). Direct comparisons confirmed that correlations between BDI-II and fatigue were significantly stronger than those between BDI-FS and fatigue (all p <.001).

The strength of the association between depression and fatigue in MS depends substantially on the depression measure used. The commonly reported strong relationship appears to be inflated by somatic item overlap in traditional depression scales. The BDI-FS provides a more specific assessment of depressive symptoms and may improve diagnostic accuracy and the interpretation of symptom relationships in MS.