Fractional excretion of potassium in the course of acute kidney injury in critically ill patients: potential monitoring tool?

OBJECTIVE: To evaluate the behavior of fractional excretion of potassium in the course of acute kidney injury in critically ill patients. METHODS: As part of a larger study in which we have evaluated blood and urinary parameters in the course of acute kidney injury, 168 patients were included. Blood and urine samples were collected daily until the removal of the urinary catheter or the initiation of renal replacement therapy. We describe the evolution of fractional excretion of potassium based on whether acute kidney injury was diagnosed, its duration (transient or persistent) and its severity (creatinine-based Acute Kidney Injury Network - AKIN stage). The diagnostic performance of fractional excretion of potassium in predicting the duration of acute kidney injury and the need for renal replacement therapy on the day of acute kidney injury diagnosis was also evaluated. RESULTS: Fractional excretion of potassium was significantly higher in persistent acute kidney injury compared to transient acute kidney injury on the day of acute kidney injury diagnosis (24.8 vs. 13.8%, p<0.001). Both groups had the median fractional excretion of potassium increasing in the two days preceding the acute kidney injury diagnosis. Patients without acute kidney injury had stable low fractional excretion of potassium values. The fractional excretion of potassium was fairly accurate in predicting persistent acute kidney injury (area under the curve: 0.712; 95% confidence interval: 0.614-0.811; p<0.001) on the day of acute kidney injury diagnosis. The area under the curve was 0.663 (95% confidence interval: 0.523-0.803; p=0.03) for renal replacement therapy. The fractional excretion of potassium increased with maximum AKIN stage reached, in both transient and persistent acute kidney injury. CONCLUSION: Sequential fractional excretion of potassium assessment appears to be useful in critically ill patients at risk for acute kidney injury.