The Fractional Excretion of Sodium (FeNa) is a calculated diagnostic tool used to assess how the kidneys are handling sodium, reflecting their current function. This calculation quantifies the percentage of sodium filtered by the kidney that is subsequently excreted in the urine. Clinicians primarily use the FeNa value to differentiate between the two most common causes of acute kidney injury (AKI): volume depletion (pre-renal azotemia) and physical damage to the kidney tubules (intrinsic renal injury).
Required Laboratory Values
The FeNa calculation requires four specific laboratory measurements taken from both the blood and a single, concurrent urine sample. These four values are the Serum Sodium (SNa), Urine Sodium (UNa), Serum Creatinine (SCr), and Urine Creatinine (UCr). The samples must be collected at approximately the same time, ideally within a two-hour window, to ensure the values reflect the same physiological state.
The need for four values stems from the principle of clearance. Creatinine, a waste product generated by muscle metabolism, is used as a surrogate marker for the overall filtration rate of the kidney. By including creatinine values, the FeNa calculation normalizes the sodium excretion to the kidney’s current filtration capacity, providing a true percentage of sodium handling. This ratio accounts for variations in urine volume and concentration that would otherwise make a simple urine sodium measurement unreliable.
The Step-by-Step Calculation Process
The Fractional Excretion of Sodium is calculated using a specific formula that combines the four required measurements into a single percentage. The formula is expressed as: FeNa (%) = [(\(\text{Urine Na} \times \text{Serum Cr}\)) / (\(\text{Serum Na} \times \text{Urine Cr}\))] \(\times\) 100. This calculation compares the clearance of sodium to the clearance of creatinine. The numerator represents the rate of sodium excretion, while the denominator represents the rate at which sodium is filtered by the kidney.
Consider a patient with the following laboratory results: Serum Sodium of 140 mEq/L, Serum Creatinine of 2.0 mg/dL, Urine Sodium of 15 mEq/L, and Urine Creatinine of 100 mg/dL. The first step involves multiplying the Urine Sodium (15) by the Serum Creatinine (2.0) to get 30 (the numerator). The second step is to multiply the Serum Sodium (140) by the Urine Creatinine (100) to get 14,000 (the denominator).
The third step calculates the ratio by dividing 30 by 14,000, resulting in 0.00214. This ratio is then multiplied by 100 to yield the FeNa percentage. In this example, the resulting FeNa is 0.21%. The units for sodium and creatinine must be consistent within the calculation, though they cancel out in the final ratio.
What the Final Percentage Indicates
The resulting FeNa percentage reflects the kidney’s physiological response to the underlying cause of acute kidney injury. A low FeNa value, typically defined as less than 1%, indicates the kidney is actively conserving sodium and water. This strong sodium reabsorption suggests the body is attempting to correct a state of low effective circulating volume, such as dehydration or heart failure. This pattern is characteristic of pre-renal azotemia, where the kidney is functionally intact but lacks sufficient blood flow.
Conversely, a higher FeNa value, usually greater than 2%, suggests the kidney tubules have lost their ability to reabsorb sodium efficiently. This failure to conserve sodium indicates damage to the kidney structure, most commonly acute tubular necrosis (ATN), a form of intrinsic renal injury. Damaged tubules allow filtered sodium to pass into the urine rather than be reclaimed, resulting in a high fractional excretion.
The range between 1% and 2% is often considered indeterminate, representing a potential overlap between pre-renal and intrinsic causes. A value in this middle range may indicate early or resolving tubular damage, or it may be seen in less common conditions.
The FeNa serves as a key piece of evidence to guide treatment decisions. A low FeNa suggests fluid resuscitation may be beneficial, while a high FeNa points toward managing a structurally damaged kidney.
Situations Where FeNa is Misleading
While the FeNa is a valuable tool, it is not universally accurate, and several clinical situations can lead to misleading results. The most significant factor is the recent use of diuretics, such as furosemide, which directly interfere with the kidney’s ability to reabsorb sodium. Diuretics artificially increase sodium excretion, causing a falsely high FeNa even when the patient is volume-depleted, mistakenly suggesting intrinsic kidney injury. For patients who have recently received diuretics, the Fractional Excretion of Urea (FEUrea) is often a more reliable alternative.
Other conditions can complicate interpretation, sometimes resulting in a low FeNa despite intrinsic injury. Certain types of acute kidney injury, like contrast-induced nephropathy or acute glomerulonephritis, can present with a low FeNa because they cause marked renal vasoconstriction that preserves the kidney’s ability to retain sodium.
Patients with chronic kidney disease (CKD) often have a baseline FeNa that is higher than normal, making traditional cutoffs unreliable for diagnosing an acute event. Initial stages of obstructive uropathy can also present with a low FeNa due to the kidney’s early attempt at sodium retention.