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

Abstract

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.

Keywords: adult intensive & critical care; human resource management; intensive & critical care.

Figure 1

Flow diagram. ICU, intensive care unit; MIMIC, multiparameter intelligent monitoring in intensive care.

Figure 2

Kaplan-Meier survival plot of mortality (p<0.0001). The death distributions of patients among six groups of serum phosphate at admission within 28 days (A) or 90 days (B) after ICU admission. G1 (red): hypophosphataemia group (<2.5 mg/L); G2 (cyan): very-low-normal phosphate group (≥2.5 mg/dL and <3.0 mg/dL); G3 (yellow): low-normal phosphate group (≥3.0 mg/dL and <3.5 mg/dL); G4 (pink): high-normal phosphate group (≥3.5 mg/dL and <4.0 mg/dL); G5 (blue): very-high-normal phosphate group (≥4.0 mg/dL and <4.5 mg/dL); G6 (purple): hyperphosphataemia group (≥4.5 mg/dL). ICU, intensive care unit.

Figure 3

Cubic spline analysis. The plot described the relationship between serum phosphate as a continuous variable and the probability of 28 days (A) or 90 days (B) mortality after ICU admission. The dashed area indicated the 95% CI. ICU, intensive care unit.