Nonlinear Association Between Admission Neutrophil Count and 14-Day Mortality in Patients with Traumatic Brain Injury: A Retrospective Cohort Study.

Traumatic brain injury (TBI) is a major cause of morbidity and mortality, and early risk assessment remains clinically important because prognostic information is often required during the emergency admission period. This study evaluated the association between admission neutrophil count and 14-day all-cause mortality in adult patients with TBI. We analyzed an institutional cohort of 1,361 adults hospitalized with TBI between 2017 and 2023. Neutrophil count was obtained from the first complete blood count performed after emergency presentation or hospital admission. Patients were categorized according to neutrophil count tertiles, and 14-day all-cause mortality was defined as the primary outcome. The association between neutrophil count and mortality was assessed using univariable and multivariable logistic regression models. Potential nonlinear relationships and threshold effects were explored using generalized additive models and two-piecewise linear regression analyses. Fourteen-day mortality increased across neutrophil count tertiles, from 0.66% in the low tertile to 1.32% in the middle tertile and 5.73% in the high tertile. In the fully adjusted model, each 1 × 109/L increase in neutrophil count was associated with higher odds of 14-day mortality (odds ratio [OR] = 1.093, 95% confidence interval [CI]: 1.003-1.190, P = 0.041). Threshold analysis identified a nonlinear association with an estimated inflection point of 3.81 × 109/L. Below this threshold, the association was inverse, whereas above it, the association was positive. These findings demonstrate an association between admission neutrophil count and 14-day all-cause mortality in patients with TBI and suggest that admission neutrophil count may serve as a readily available marker for early risk stratification. The threshold finding should be interpreted as exploratory.