Leukocyte Morphology Changes during Preseason in Elite Soccer Players: A Pilot Study.

Monitoring internal physiological responses in elite soccer players remains challenging due to the limitations of subjective scales and nonspecific biomarkers. This study examined whether leukocyte morphology and endothelial stress markers respond to acute exercise and a 6-week preseason training program in elite soccer players and explored their potential utility for objective athlete monitoring. Twenty-two male outfield players were assessed at baseline (48 h pre-preseason), immediately after a maximal Yo-Yo intermittent running test, and 72 hours after the preseason period. Leukocyte volume, conductivity, and scatter, factor VIII, von Willebrand factor, inflammatory, and metabolic markers were measured. Principal component analysis and K-means clustering explored morphological response patterns. Acute exercise increased white blood cells (+29%), neutrophils (+47%), platelets (+15%), factor VIII (+102%), and von Willebrand factor (+80%; all p<0.003) without changes in volume, conductivity, and scatter parameters. After 6 weeks, the lymphocyte volume, conductivity, and scatter and neutrophil conductivity and scatter decreased significantly (all p<0.0042), whereas leukocyte counts remained unchanged. Cluster analysis identified two morphological response patterns. These findings indicate phase-specific leukocyte morphological and endothelial responses to acute and chronic training and suggest that automated hematology markers may detect subclinical physiological adaptations not captured by conventional indices.