Day-Specific Associations of Serum Phosphate With 90-Day Mortality in Adult Patients With sepsis: A Landmark Analysis.

INTRODUCTION: Serum phosphate abnormalities are common in sepsis, yet associations with outcomes remain inconsistent. We examined day-specific associations between serum phosphate levels during the first week after intensive care unit (ICU) admission and 90-day mortality. METHODS: This single-center retrospective study included adults meeting Sepsis-3 criteria admitted to the ICU between April 2011 and March 2021 with at least one serum phosphate measurement on Days 1-7. Phosphate was categorized as low (≤ 0.84 mmol/L [≤ 2.6 mg/dL]), normal (0.87-1.45 mmol/L [2.7-4.5 mg/dL]), or high (≥ 1.49 mmol/L [≥ 4.6 mg/dL]). Day-specific landmark Cox models assessed 90-day mortality using the normal group as reference and adjusted for age, sex, body mass index, arterial pH, daily Sequential Organ Failure Assessment (SOFA) score excluding the central nervous system component, renal replacement therapy status, and previous-day phosphate supplementation on Days 2-7. Day-specific logistic models evaluated factors associated with phosphate abnormalities. RESULTS: Among 480 patients, hyperphosphatemia was consistently associated with higher 90-day mortality across Days 1-7 after adjustment. Hypophosphatemia showed phase-dependent associations that attenuated after adjustment. Arterial pH was most strongly associated with phosphate abnormalities early, whereas SOFA score showed more consistent associations later. CONCLUSION: Hyperphosphatemia during the first ICU week was consistently associated with increased 90-day mortality, whereas associations with hypophosphatemia were weaker after adjustment. The prevalence and clinical correlates of phosphate abnormalities varied across days, supporting interpretation by measurement timing and clinical severity.