Osmotic diuresis due to urea as the cause of hypernatraemia in critically ill patients

Background. Hypernatraemia is common in critically ill patients and has been shown to be an independent predictor of mortality. Osmotic urea diuresis can cause hypernatraemia due to significant water losses but is often not diagnosed. Free water clearance (FWC) and electrolyte free water clearance (EFWC) were proposed to quantify renal water handling. We aimed to (i) identify patients with hypernatraemia due to osmotic urea diuresis and (ii) investigate whether FWC and EFWC are helpful in identifying renal loss of free water. Methods. In this retrospective study, we screened a registry for patients, who experienced intensive care unit (ICU)-acquired hypernatraemia. Among them, patients with hypernatraemia due to osmotic urea diuresis were detected by a case-by-case review. Total fluid and electrolyte balances together with FWC and EFWC were calculated for days of rising serum sodium and stable serum sodium. Results. We identified seven patients (10% of patients with ICU-acquired hypernatraemia) with osmotic diuresis due to urea. All patients were intubated during development of hypernatraemia and received enteral nutrition. The median highest serum sodium level of 153 mmol (Q1: 151-Q3: 155 mmol/L) was reached after a 5-day period of rise in serum sodium. During this period, FWC was -904 mL/day (Q1: -1574-Q3: -572), indicating renal water retention, while EFWC was 1419 mL/day (Q1 : 1052-Q3: 1923), showing renal water loss. While FWC did not differ between time of stable serum sodium and development of hypernatraemia, EFWC was significantly higher during rise in serum sodium. Conclusion. Osmotic urea diuresis is a common cause of hypernatraemia in the ICU. EFWC was useful in the differential diagnosis of polyuria during rising serum sodium levels, while FWC was misleading.