In a patient with an elevated serum creatinine of 1.72 mg/dL and a fractional excretion of sodium of 1.8%, what type of acute kidney injury is present?

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Last updated: September 19, 2026 • View editorial policy

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Acute Tubular Necrosis (Intrinsic Renal Injury)

With a creatinine of 1.72 mg/dL and FeNa of 1.8%, this patient most likely has intrinsic acute kidney injury, specifically acute tubular necrosis (ATN), rather than prerenal azotemia.

Interpretation of the Fractional Excretion of Sodium

The FeNa of 1.8% is the critical discriminating value here. The traditional teaching uses FeNa >1% to indicate intrinsic renal injury (ATN) and FeNa <1% to suggest prerenal azotemia 1, 2, 3. Your patient's value of 1.8% clearly exceeds this threshold, pointing toward tubular dysfunction where the kidneys have lost their ability to avidly reabsorb sodium.

Key Diagnostic Thresholds:

  • FeNa <1%: Suggests prerenal AKI (intact tubular function, avid sodium reabsorption)
  • FeNa >1%: Suggests intrinsic renal injury/ATN (impaired tubular function)
  • Your patient's FeNa = 1.8% falls definitively in the ATN range

Supporting Evidence and Nuances

Recent research confirms that UNa and FeNa show high specificity (>85%) for differentiating prerenal from intrinsic AKI 1. In a 2023 study of 431 hospitalized patients with AKI, these indices proved reliable and were not confounded by common medications like loop diuretics or ACE inhibitors, contrary to older concerns 1.

Important Caveats:

The FeNa is not perfect—there are well-documented exceptions 3:

  • Low FeNa (<1%) can occur in ATN in certain settings: contrast nephropathy, rhabdomyolysis, sepsis-induced ATN, hepatorenal syndrome, and acute glomerulonephritis
  • High FeNa can occur in prerenal states when patients are on diuretics 2

However, in the absence of these specific confounding factors, your patient's FeNa of 1.8% is highly specific for intrinsic injury.

Clinical Context Matters

The distinction between transient (prerenal) and intrinsic AKI has prognostic implications. Studies show that transient AKI still carries significant mortality (13-15%) compared to no AKI (3-4%), but persistent/intrinsic AKI has even higher mortality (29%) 4, 5.

In septic patients specifically, combined high FeNa (>0.36%) and high FEUrea (>31.5%) has a 92-94% negative predictive value for intrinsic AKI, meaning if both are elevated, intrinsic injury is very likely 5, 6. Your patient's FeNa of 1.8% fits this pattern.

Practical Approach

To confirm intrinsic AKI in your patient, also check:

  • Urine sodium: Should be >40 mEq/L in ATN (vs <20 mEq/L in prerenal)
  • Urine osmolality: Should be <350 mOsm/kg in ATN (vs >500 in prerenal)
  • Urine specific gravity: Should be <1.015 in ATN (vs >1.020 in prerenal)
  • Renal Failure Index (RFI): Should be >2 in ATN (vs <1 in prerenal) 1

Look for the underlying cause of ATN:

  • Nephrotoxic medications (aminoglycosides, NSAIDs, contrast)
  • Prolonged hypotension or ischemia
  • Sepsis
  • Rhabdomyolysis or hemolysis

Monitor for recovery: Unlike prerenal AKI which reverses rapidly with volume resuscitation, ATN typically requires 7-21 days for tubular regeneration. The creatinine trajectory over the next 48-72 hours will help confirm the diagnosis—persistent or worsening creatinine despite adequate perfusion confirms intrinsic injury 5.

References

6
Research

Acute kidney injury in sepsis: transient or intrinsic?

Critical care (London, England), 2013

Related Questions

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