Prerenal AKI vs Intrinsic AKI
How to tell Prerenal AKI and Intrinsic AKI apart when a board question makes them look alike.
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Urine sediment with muddy brown granular casts is the fastest discriminator for intrinsic AKI.
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Low effective renal perfusion with concentrated urine and rapid creatinine rise suggests prerenal AKI.
Tubular injury with active urine sediment and impaired concentrating ability supports intrinsic AKI, especially acute tubular necrosis.
Renal hypoperfusion triggers sodium and water avidity to preserve intravascular volume and GFR.
Ischemic or nephrotoxic tubular epithelial damage causes loss of reabsorptive function and tubular cast formation.
Prerenal AKI typically shows BUN to creatinine ratio above 20:1, urine sodium below 20 mEq/L, FeNa below 1%, and bland sediment with hyaline casts.
Intrinsic AKI from ATN often shows FeNa above 2%, urine sodium above 40 mEq/L, urine osmolality below 350 mOsm/kg, and muddy brown granular casts.
The trap
A common mistake is overreliance on FeNa after diuretic exposure, which can falsely elevate sodium excretion and mimic intrinsic disease. In that setting, urine sediment and response to volume assessment are more reliable.
The repair move
Next best step: assess volume status and hemodynamics immediately, then use urine sediment and chemistries to classify injury. If prerenal findings predominate, restore perfusion with isotonic fluids and treat the cause. If intrinsic ATN is present, stop nephrotoxins, avoid fluid overload, dose-adjust medications, and monitor for dialysis indications such as refractory hyperkalemia, severe acidosis, or pulmonary edema.
Try a free question
CoreA 70-year-old man has vomiting-related hypotension and creatinine increase from 0.9 to 1.8 mg/dL. Urine sodium is 12 mEq/L, FeNa 0.4%, BUN to creatinine ratio 28:1, and sediment is bland. What is the most likely diagnosis?
Show answer and explanation
Answer: A. Prerenal AKI. Low FeNa, high BUN to creatinine ratio, and bland sediment in a hypovolemic context indicate prerenal AKI.
Why the other choices are tempting
- B. Wrong because ATN usually has higher FeNa and muddy brown casts, not bland sediment.
- C. Wrong because interstitial nephritis typically includes pyuria, white cell casts, and often drug exposure features.
- D. Wrong because no obstruction clues or hydronephrosis findings are provided.
- E. Wrong because glomerulonephritis often presents with hematuria, proteinuria, and red cell casts.
Flashcards
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Which urine microscopy finding points strongly to intrinsic ATN rather than prerenal AKI?
Muddy brown granular casts indicate tubular epithelial injury and support intrinsic AKI.
What classic FeNa threshold supports prerenal AKI in the right context?
FeNa below 1% supports prerenal physiology when diuretics are not confounding interpretation.
Quick answers
What is the fastest way to tell Prerenal AKI from Intrinsic AKI?
Urine sediment with muddy brown granular casts is the fastest discriminator for intrinsic AKI.
What is the key clue for Prerenal AKI?
Low effective renal perfusion with concentrated urine and rapid creatinine rise suggests prerenal AKI.
What is the key clue for Intrinsic AKI?
Tubular injury with active urine sediment and impaired concentrating ability supports intrinsic AKI, especially acute tubular necrosis.
What is the common trap with Prerenal AKI vs Intrinsic AKI?
A common mistake is overreliance on FeNa after diuretic exposure, which can falsely elevate sodium excretion and mimic intrinsic disease. In that setting, urine sediment and response to volume assessment are more reliable.