Change trends in serum phosphate levels predict in-hospital mortality in critically ill septic patients

Serum phosphate levels are strongly correlated with the prognosis of septic patients. However, previous studies have concentrated on individual phosphate levels, and the relationship between change trends in serum phosphate levels and in-hospital mortality has seldom been reported. We aimed to investigate whether the level and change trends of serum phosphate were associated with in-hospital mortality. We classified patients using k-means clustering analysis into clusters with changes in serum phosphate levels and used logistic regressions to explore the relationships between different clusters and in-hospital mortality, taking the cluster with the smallest change as a reference. Restricted cubic spline regression was used to examine the shape of the correlation between changes in serum phosphate levels and in-hospital mortality. Subgroup analyses and interaction analyses were performed to discover potential impact factors. A total of 1810 (21.1%) of 8586 participants died during their hospital stay. After adjustment for baseline variables, cluster 2 (OR 1.303, 95% CI 1.101-1.542, p = 0.002), cluster 3 (OR 1.348, 95% CI 1.158-1.57, p < 0.001), cluster 4 (OR 1.652, 95% CI 1.225-2.222, p = 0.001) and cluster 5 (OR 2.745, 95% CI 2.212-3.407, p < 0.001) remained associated with significantly increased mortality. The changes in serum phosphate levels and in-hospital mortality were linear according to restricted cubic spline regression. According to the subgroup analyses, the ORs of the female subgroup and mechanical ventilation subgroup were lower than those of their counterparts across all clusters. Multiplicative and additive interactions were detected between phosphate clusters and mechanical ventilation. First, a high and unstable serum phosphate level is associated with increased mortality in septic patients. Second, for those with elevated phosphate levels, treatments to lower serum phosphate may reduce mortality in septic patients. Third, an increasing trend in phosphate levels may be more important than a high level in predicting poor prognosis in septic patients.