In an 18‑year‑old woman with severe volume depletion and pre‑renal acute kidney injury, which laboratory finding is most consistent: BUN/creatinine ratio 13, fractional excretion of sodium 2.3 %, urine osmolality 300 mOsm/kg, urine sodium 13 mEq/L, or fractional excretion of urea 40 %?

Urine Sodium of 13 mEq/L is Most Consistent with Prerenal AKI

In this 18-year-old with severe volume depletion from vomiting and diarrhea, a urine sodium of 13 mEq/L is the laboratory finding most consistent with prerenal acute kidney injury. This value reflects appropriate renal sodium conservation in response to hypovolemia, which is the hallmark physiologic response in prerenal states 1, 2.

Why Urine Sodium 13 mEq/L is Correct

• Urine sodium <10-20 mEq/L is highly specific (>85%) for prerenal AKI, indicating intact tubular function with avid sodium reabsorption in response to volume depletion 2.
• This patient's clinical presentation—persistent vomiting, diarrhea, dry mucous membranes, poor skin turgor, tachycardia, and thready pulses—represents classic severe volume depletion causing prerenal azotemia 1.
• The kidneys respond to hypovolemia by maximally conserving sodium, resulting in very low urinary sodium concentrations when tubular function remains intact 1, 3.

Why the Other Options Are Incorrect

BUN/Creatinine Ratio of 13

• A BUN/Cr ratio of 13 is actually LOW and inconsistent with prerenal AKI 1.
• Prerenal states typically produce BUN/Cr ratios >20:1 due to enhanced proximal tubular urea reabsorption in volume-depleted states 1.
• The 2022 guidelines do not even mention BUN/Cr ratio as a diagnostic criterion for differentiating prerenal from intrinsic AKI, reflecting its limited clinical utility 1.

Fractional Excretion of Sodium (FENa) of 2.3%

• FENa >2% strongly suggests intrinsic renal injury (acute tubular necrosis), not prerenal AKI 1, 4.
• In prerenal states, FENa should be <1%, reflecting avid tubular sodium reabsorption 1, 4.
• In oliguric patients without CKD or diuretics, FENa <1% has 95% sensitivity and 91% specificity for prerenal AKI 4.
• This patient's FENa of 2.3% would indicate tubular dysfunction inconsistent with simple volume depletion 3, 2.

Urine Osmolality of 300 mOsm/kg

• Urine osmolality of 300 mOsm/kg is inappropriately LOW for prerenal AKI 5.
• In prerenal states with intact tubular function, the kidneys should concentrate urine to >500 mOsm/kg in response to volume depletion 2.
• A urine osmolality of only 300 mOsm/kg suggests impaired concentrating ability, which is pathognomonic for tubular dysfunction (ATN), not prerenal physiology 6.

Fractional Excretion of Urea (FEUrea) of 40%

• FEUrea >35% indicates intrinsic renal injury (ATN), not prerenal AKI 1, 7.
• Prerenal AKI should demonstrate FEUrea <35%, reflecting enhanced urea reabsorption in volume-depleted states 1, 7.
• In a meta-analysis of 915 patients, FEUrea >35% had 66% sensitivity and 75% specificity for intrinsic AKI 7.
• FEUrea of 40% is inconsistent with the preserved tubular function expected in simple volume depletion 1.

Clinical Interpretation Algorithm

For this patient with clear volume depletion:

1. Urine sodium <20 mEq/L confirms prerenal physiology with intact tubular sodium conservation 1, 2.
2. FENa should be <1% in prerenal AKI (not 2.3% as listed) 1, 4.
3. Urine osmolality should be >500 mOsm/kg (not 300) in prerenal states 2.
4. FEUrea should be <35% (not 40%) in prerenal AKI 1, 7.
5. BUN/Cr ratio should be >20:1 (not 13) in prerenal azotemia 1.

Important Caveats

• If this patient had received diuretics, FENa would be unreliable, as diuretics increase urinary sodium excretion even in prerenal states 3, 4.
• In diuretic-treated patients, FEUrea becomes more useful than FENa, though even FEUrea has reduced sensitivity (52%) in this setting 7.
• The presence of oliguria significantly improves the diagnostic accuracy of these indices—in oliguric patients without CKD or diuretics, FENa <1% achieves 95% sensitivity and 91% specificity for prerenal AKI 4.