Nonlinear Associations of Uric Acid and Mitochondrial DNA with Mortality in Critically Ill Patients

Background: Mitochondrial DNA (mtDNA) has strong pro-inflammatory potential and was found to be associated with mortality in critically ill patients. The purine bases from circulating cell-free DNA, including mtDNA, are catabolised into uric acid, contributing to elevated systemic levels. However, the prognostic value of uric acid in unselected critically ill intensive care unit (ICU) patients remains unclear. We aimed to investigate the association between uric acid levels at admission and 30-day mortality, assess its correlation with mtDNA, and examine prognostic relevance based on the primary cause of admission. Methods: This prospective single-centre study included 226 patients admitted to a tertiary care ICU. Uric acid and mtDNA levels were assessed at admission. Survival analyses were performed in the overall cohort and in subgroups stratified by primary diagnosis. Results: Uric acid showed a U-shaped association with 30-day mortality, with both low and high levels linked to reduced survival. In multivariate analysis, the 4th quartile of uric acid remained associated with adverse outcomes, independent of sex, vasopressors, mechanical ventilation, and creatinine (HR 2.549, 95% CI: 1.310–4.958, p = 0.006). A modest correlation was observed between uric acid and mtDNA (r = 0.214, p = 0.020). However, prognostic relevance varied by diagnosis. While uric acid predicted mortality in patients following cardiac arrest (p = 0.017), mtDNA was found to bear prognostic value in cardiogenic shock and decompensated heart failure (p = 0.009). Conclusions: Uric acid was independently associated with mortality in critically ill patients, with both low and high levels carrying prognostic value. Its predictive capabilities differed from mtDNA but showed partial overlap. However, both markers exhibited varying prognostic performance depending on the primary cause of admission.