Abstract
<title>Abstract</title> <p> Background Existing studies have demonstrated serum phosphate levels are associated with the poor prognosis of acute kidney injury (AKI) in intensive care units (ICUs). Nevertheless, these studies only involved single measurements of phosphate levels and never explored the prognostic impact of dynamic changes in phosphate levels. Accordingly, this study delved into the impact of changes in serum phosphate trajectories on 28-day mortality in patients with AKI in the ICU. Methods This retrospective cohort study was conducted based on the data of patients with AKI from the Medical Information Mart for Intensive Care-IV v2.2 database. Based on at least three measurements of serum phosphate levels within five days of ICU admission, heterogeneous serum phosphate trajectories among AKI patients were analyzed with group-based trajectory modeling. The survival of different trajectory groups was compared with Kaplan-Meier survival curves. The association between serum phosphate trajectories and the risk of 28-day mortality was assessed with Cox regression models, and the relationship between serum phosphate levels and prognosis was evaluated with a restricted cubic spline (RCS) analysis. Finally, differences between subgroups were analyzed with a subgroup analysis. Results This study included 21,353 AKI patients, among which 2,808 (13.2%) patients died within 28 days. Four serum phosphate trajectories were defined, namely hypophosphatemia-increasing trend (Trajectory 1), normal level-steady trend (Trajectory 2), hyperphosphatemia-decreasing trend (Trajectory 3), and hyperphosphatemia-persistent trend (Trajectory 4). The comparative analysis of the four trajectories revealed that compared with Trajectory 1, Trajectory 2 (hazard ratio [HR] = 0.84, 95% confidence interval [CI] = 0.74–0.95, <italic>P</italic> = 0.04) was significantly associated with reduced mortality, whilst Trajectory 3 (HR = 1.14, 95%CI = 1.10–1.24, <italic>P</italic> < 0.001) and trajectory 4 (HR = 1.21, 95%CI = 1.05–1.37, <italic>P</italic> < 0.001) were linked to increased mortality. RCS curves displayed a U-shaped relationship between serum phosphate levels at ICU admission and 28-day mortality. Furthermore, it was found that the serum phosphate trajectory interacted only with sepsis (interaction <italic>P</italic> < 0.001) after subgroup analyses for Age, Gender, Hypertension, Sepsis, Continuous Renal Replacement Therapy(CRRT), and Mechanical Ventilation(MV). Conclusion Identifying different serum phosphate trajectories can assist in distinguishing AKI subtypes with the risk of poor prognosis, thereby fostering the development of individualized management strategies. </p>