Differential associations between white blood cell counts and fatigue in young and older adults

Background and aims: The aims of this exploratory study were to study whether fatigue might be related to the cellular immune system by 1) analysing if the number of white blood cell subsets are related to fatigue and 2) if possible relationships vary in younger and older community-dwelling individuals. Methods: The participants were recruited from nine general practitioners in Aarhus County, Denmark and included 196 individuals aged 20–35 years and 314 individuals aged 70–85 years. The white blood cell counts included number of total leukocytes, lymphocytes, monocytes, neutrophils, eosinophils, and basophils. General fatigue was measured by a question from the SF-12 Vitality-Scale and mobility-related fatigue by the Avlund Mob-T Scale. Results: Total number of lymphocytes was associated with fatigue in the old sample, both in the crude and adjusted analyses. Total number of leukocytes and netrophils were associated with fatigue in both age groups in the crude analyses. In the old sample the estimates were attenuated to non-significance when adjusting for physical activity and disability. In the young sample the estimates stayed statistically significant in the fully adjusted analyses regarding number of neutrophils, while the associations between fatigue and number of leukocytes were attenuated to non-significance when adjusting for depressive mood. Conclusion: Total number of leukocytes, lymphocytes and neutrophils were associated with fatigue in both age groups, while the explanatory factors for the associations differed by age group, in that the associations were partly explained by physical activity and disability in the old sample and partly by depressive mood in the young sample. The findings provide initial insight into the potential role of leukocyte, neutrophil and lymphocyte counts in the development of fatigue.