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Background: Acute renal failure (ARF) can be triggered or aggravated by reactive oxygen species (ROS) but established ARF per se might also affect the antioxidant defence mechanisms of the organism. We evaluated a broad pattern of antioxidants in critically ill patients with multiple organ failure with (MOF-ARF) and without acute renal failure (MOF) to identify any potential involvement of renal dysfunction in the depletion of the antioxidant system.Methods: Observational study; 13 patients with MOF were investigated (9 with and 4 without ARF), and 17 healthy subjects served as controls. Blood samples were drawn after establishment of MOF. Plasma levels of ascorbate, alpha-tocopherol, retinol, beta-carotene, selenium and lipid peroxidation products (MDA) were determined and the activities of the antioxidant enzymes catalase (CAT), superoxide dismutase (SOD) and glutathione-peroxidase (GSH-PX) in erythrocytes were measured. In addition, ROS production (superoxide anion and hydrogen peroxide) in activated neutrophils was assessed.Results: Plasma levels of ascorbate, beta-carotene and selenium were reduced in both patient groups, when compared to controls. Decrease in selenium was more pronounced in the MOF-ARF group. MDA levels were increased in both groups, again being more pronounced in MOF-ARF patients compared to MOF patients. Hydrogen peroxide production by neutrophils from both patient groups was lower than from controls.Conclusion: A depression of the antioxidative system is present in critically ill patients with MOF. In patients with associated ARF this was even more pronounced and plasma MDA levels were higher, suggesting an additional effect on the antioxidative potential in the presence of renal dysfunction and/or renal replacement therapy. [ABSTRACT FROM AUTHOR]