What are the uses of Fractional Excretion of Sodium (FeNa) and urine creatinine in clinical practice?

Clinical Applications of FeNa and Urine Creatinine

Fractional excretion of sodium (FeNa) and urine creatinine are essential diagnostic tools for differentiating causes of acute kidney injury, with FeNa particularly valuable in distinguishing prerenal azotemia from acute tubular necrosis, while urine creatinine serves as a critical component in calculating renal clearance and assessing kidney function. 1

Fractional Excretion of Sodium (FeNa)

Primary Uses:

• Differentiates between causes of acute kidney injury (AKI), particularly in oliguric patients 1
• Assesses sodium handling by the kidneys to determine if low urine output is due to volume depletion or intrinsic kidney damage 1
• Helps distinguish between prerenal azotemia and acute tubular necrosis (ATN) 1, 2

Interpretation:

• FeNa <1% suggests prerenal causes of AKI, including hypovolemia and hepatorenal syndrome (HRS) 1, 2
• FeNa >1% suggests structural causes of AKI like acute tubular necrosis 1, 2
• In cirrhosis, FeNa <1% has high sensitivity (100%) but poor specificity (14%) for prerenal causes 1

Limitations:

• Affected by recent diuretic use, which increases sodium excretion regardless of underlying cause 1, 3
• May be affected by water reabsorption in the kidneys 1
• Less reliable in certain populations, including patients with cirrhosis 1

Fractional Excretion of Urea (FEUrea)

Advantages over FeNa:

• Less influenced by diuretic therapy as urea is mainly reabsorbed in proximal tubules and collecting ducts 1, 3
• Better discriminates hepatorenal syndrome from prerenal azotemia or ATN 1
• FEUrea <28.16% had sensitivity of 75% and specificity of 83% in separating HRS from non-HRS 1
• In patients receiving diuretics, FEUrea <35% has higher sensitivity and specificity than FeNa for differentiating prerenal from intrinsic AKI 3, 4

Urine Creatinine

Clinical Applications:

• Essential component for calculating creatinine clearance, which estimates glomerular filtration rate (GFR) 1
• Used in the calculation of FeNa, FEUrea, and other fractional excretion measurements 1
• Serves as a denominator to standardize urinary biomarker concentrations (e.g., NGAL/creatinine ratio) 1
• Helps assess kidney function in patients with heart failure and cirrhosis 1

Interpretation in Clinical Practice:

• 24-hour urine creatinine collection provides information about muscle mass and adequacy of collection 1
• Urine creatinine is used to normalize urinary biomarkers to account for variations in urine concentration 1, 5
• Low urine creatinine may indicate inadequate collection or severe muscle wasting 1

Combined Use in Specific Clinical Scenarios

Acute Kidney Injury in Cirrhosis:

• FeNa and FEUrea help differentiate hepatorenal syndrome from other causes of AKI 1
• FEUrea is more reliable than FeNa in patients with cirrhosis, especially those on diuretics 1
• Combination with other biomarkers (NGAL, KIM-1) improves diagnostic accuracy 1, 5

Heart Failure:

• FeNa helps assess sodium avidity and response to diuretics 1
• Urine sodium concentration <50-70 mEq/L after loop diuretics may reflect heightened kidney sodium avidity 1
• Maintaining transkidney perfusion pressure >60 mmHg is important for kidney function 1

Sepsis:

• Combined high FeNa and high FEUrea strongly predicts transient AKI 6
• Combined high FeNa and low FEUrea is associated with intrinsic AKI 6
• Oliguria is an earlier sign of impending AKI than increase in serum creatinine in sepsis 6

Pitfalls and Caveats

• FeNa has limited utility in patients receiving diuretics - consider FEUrea instead 3, 4
• In cirrhosis, standard cutoffs for FeNa may be less reliable due to altered sodium handling 1
• Single measurements may be less valuable than trends over time 5
• Interpretation should consider clinical context, including volume status and medication use 1
• Newer biomarkers (NGAL, KIM-1, IL-18) may provide additional prognostic information when combined with traditional markers 1, 5