Association between serum phosphate and mortality in critically ill patients: a large retrospective cohort study.

OBJECTIVES This research aims to explore the impact of serum phosphate on the mortality of critically ill patients. DESIGN A retrospective large cohort study. SETTING Our data were extracted from a publicly accessible database named 'Multiparameter Intelligent Monitoring in Intensive Care Database III'. PARTICIPANTS 27 131 patients were included by clear definitions of selection and exclusion criteria. INTERVENTIONS We used initial phosphate at admission as a design variable. Patients were divided into six groups with different serum phosphate levels and five groups at different intensive care unit (ICU) departments. PRIMARY AND SECONDARY OUTCOMES 28-day and 90-day mortality were primary outcomes. All-cause mortality and length of stay ICU were secondary outcomes. RESULTS Patients with very-high-normal serum phosphate, hypophosphataemia and hyperphosphataemia had worse outcomes. And the relationship between serum phosphate and the probability of 28-day or 90-day mortality had a linear relationship. After adjustment for potential confounders, hypophosphataemia and hyperphosphataemia were not significantly associated with 28-day or 90-day mortality. Nevertheless, at the medical ICU, hyperphosphataemia was associated with increased 28-day or 90-day mortality (HR=0.64, 95% CI 0.48 to 0.84, p=0.0017; HR=0.72, 95% CI 0.57 to 0.91, p=0.0067, respectively), using group 2 (≥2.5 mg/dL and <3.0 mg/dL) as the reference group. CONCLUSIONS Patients with very-high-normal serum phosphate also had worse outcomes, it might be necessary to re-evaluate the definitions of the normal reference range for serum phosphate. Hypophosphataemia and hyperphosphataemia are not the independent risk factors of 28-day or 90-day ICU mortality, which leads us to consider whether phosphate monitoring is not a necessary measure in critically ill patients. But hyperphosphataemia was associated with increased 28-day or 90-day mortality at the medical ICU, which emphasises the potential importance of early diagnosis and treatment of hyperphosphataemia for the patients who were admitted to the medical ICU.