What does a Fractional Excretion of Sodium (FENA) of 0.779 indicate and how should it be managed?

From the Guidelines

A Fractional Excretion of Sodium (FENA) of 0.779 indicates a prerenal cause of acute kidney injury, and management should focus on addressing the underlying cause of decreased renal perfusion. The FENA value of 0.779 is below 1%, suggesting that the kidneys are functioning properly but are receiving inadequate blood flow, which is a characteristic of prerenal acute kidney injury 1. This condition should be managed by addressing the underlying cause of decreased renal perfusion through fluid resuscitation, typically with isotonic crystalloids like normal saline or lactated Ringer's solution. Some key points to consider in management include:

• Initial bolus of 500-1000mL followed by maintenance fluids should be administered while monitoring vital signs, urine output, and electrolytes.
• Any medications potentially causing renal hypoperfusion (such as NSAIDs, ACE inhibitors, or ARBs) should be temporarily discontinued.
• Common causes of prerenal injury include volume depletion from vomiting, diarrhea, bleeding, heart failure, liver disease, or sepsis - identifying and treating the specific cause is essential for complete resolution.
• If the patient doesn't respond to initial fluid resuscitation, further hemodynamic assessment may be necessary, as indicated by the patient's clinical presentation and response to treatment 1. It is essential to note that the provided study 1 focuses on the management of adult patients with ascites due to cirrhosis, which may not directly relate to the FENA value of 0.779, but the principles of addressing underlying causes of decreased renal perfusion and fluid resuscitation are applicable in the context of prerenal acute kidney injury.

From the Research

Interpretation of FENA

• A Fractional Excretion of Sodium (FENA) of 0.779 is less than 1%, which suggests that the patient may have prerenal azotemia or volume depletion 2.
• According to a study published in 1985, a FENA of 0.4% to 0.8% is consistent with volume depletion in a patient with acute tubular necrosis 3.
• Another study published in 2010 found that FENA values were lower in prerenal failure patients (0.33 +/- 0.57) compared to acute tubular necrosis patients (4.74 +/- 6.12) 4.

Management

• The management of a patient with a FENA of 0.779 would depend on the underlying cause of the acute renal failure.
• If the patient has prerenal azotemia or volume depletion, treatment would focus on correcting the volume depletion and addressing any underlying causes 3, 2.
• A study published in 1985 found that vigorous intravenous fluid therapy in a patient with acute tubular necrosis and volume depletion produced a prompt increase in urine volume and improvement in renal function 3.

Differential Diagnosis

• A FENA of less than 1% can help differentiate prerenal azotemia from acute tubular necrosis 5, 2.
• However, it is essential to consider other factors, such as urinary output and serum creatinine levels, to determine the underlying cause of acute renal failure 6.
• A combination of FENA and fractional excretion of urea (FEUrea) can provide additional information to distinguish between transient and intrinsic acute kidney injury 6.