From the Guidelines
A fractional excretion of sodium (FENa) of 2.5% indicates intrinsic renal failure, most likely acute tubular necrosis (ATN), as it suggests that the kidneys are unable to properly reabsorb sodium, a hallmark of intrinsic kidney damage. This interpretation is based on the understanding that in normal functioning kidneys or in pre-renal causes of kidney injury, the FENa is typically less than 1% as the kidneys attempt to conserve sodium 1. When the FENa exceeds 1%, it points towards structural causes of acute kidney injury (AKI) like ATN. Although a recent study mentioned in the evidence from 1 reported no correlation between the FENa and the etiology of AKI on kidney biopsy in patients on a liver transplant waiting list, the general principle that a high FENa suggests intrinsic renal damage remains a guiding principle in clinical practice.
The management of a patient with a FENa of 2.5% would thus focus on identifying and addressing the underlying cause of the intrinsic renal failure, which could include nephrotoxic medications, sepsis, or prolonged ischemia, rather than simply providing volume repletion as would be appropriate for pre-renal causes 1. It's also important to consider additional laboratory values like the BUN/creatinine ratio, urine sediment examination, and clinical context for a comprehensive assessment of kidney function and appropriate management. The use of diuretics and their potential side effects, as discussed in 1, should also be carefully evaluated in the context of managing patients with ascites and potential renal impairment, emphasizing the need for a balanced approach to fluid management and diuretic use.
Key considerations in the interpretation of FENa include:
- The FENa reflects sodium handling more accurately than urinary sodium alone but is affected by water reabsorption 1.
- A FENa cutoff of <1% suggests pre-renal causes, including hepatorenal syndrome (HRS), and FENa >1% is suggestive of structural causes of AKI like ATN 1.
- Fractional excretion of urea (FEUrea) may better discriminate HRS from pre-renal azotemia or ATN, especially in patients with cirrhosis 1.
From the Research
Fractional Excretion of Sodium Interpretation
- A fractional excretion of sodium (FENa) of 2.5% is higher than the typical values associated with volume depletion or prerenal causes of acute kidney injury, which are usually less than 1% 2, 3, 4.
- However, it is lower than the values typically seen in acute tubular necrosis, which are often greater than 3% 2.
- The interpretation of FENa must be done in the context of the patient's clinical setting, as it can be influenced by various factors such as diuretic use, chronic kidney disease, and sodium-avid states 3, 5, 6.
- A FENa of 2.5% may suggest intrinsic renal injury, but it is not diagnostic on its own and should be considered in conjunction with other laboratory and clinical findings 3, 5, 6.
- The diagnostic performance of FENa for differentiating intrinsic from prerenal acute kidney injury is limited in patients with chronic kidney disease or those on diuretic therapy, but it may be more useful in oliguric patients without these confounding factors 6.