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.