Bit · Renal
Acute Kidney Injury — Prerenal vs Intrinsic vs Postrenal
AKI splits at the start: is the kidney getting blood, working properly, or able to drain? Three categories, three different labs.
Mechanism
AKI is a rapid (< 7 days) rise in creatinine and/or fall in urine output. Three causes:
- Prerenal — kidney is structurally fine but underperfused. Causes: hypovolemia (hemorrhage, dehydration, sepsis early, third-spacing), low cardiac output, renal artery stenosis, hepatorenal syndrome, NSAIDs (afferent constriction), ACEi (efferent dilation). Kidneys respond appropriately to low flow: maximally reabsorb Na and water. Reverses with restoration of perfusion.
- Intrinsic — kidney parenchyma is damaged. Subtypes:
- Acute tubular necrosis (ATN) — most common; from prolonged ischemia OR nephrotoxins (aminoglycosides, contrast, myoglobin, cisplatin, amphotericin). Brown muddy casts.
- Acute interstitial nephritis (AIN) — drug hypersensitivity (NSAIDs, PPIs, β-lactams, sulfa, diuretics). Fever, rash, eosinophilia, eosinophiluria, WBC casts.
- Glomerulonephritis — see nephritic Bit. RBC casts.
- Vascular — vasculitis, malignant HTN, TTP/HUS, atheroembolic disease.
- Postrenal — obstruction to urinary outflow. Causes: BPH (most common cause in older men), nephrolithiasis (bilateral or single kidney), pelvic malignancy, neurogenic bladder. Reversible if treated promptly.
Differentiator Table
| Prerenal | Intrinsic (ATN) | Postrenal | |
| Mechanism | Underperfusion | Tubular cell death | Outflow obstruction |
| BUN:Cr ratio | > 20:1 | < 15:1 | Variable (high early, low late) |
| Urine Na | < 20 mEq/L | > 40 mEq/L | Variable |
| FENa | < 1% | > 2% | Variable |
| Urine osmolality | > 500 mOsm/kg | ≈ 300 (isosthenuria) | Variable |
| Casts | Hyaline | Muddy brown granular | None specific |
| Imaging | Normal | Normal initially | Hydronephrosis |
| Reversibility | Quick with fluid | Days to weeks | Quick if obstruction relieved |
The Pivot
Three labs do most of the work:
- BUN:Cr ratio. > 20:1 → prerenal. < 15:1 → intrinsic.
- FENa (fractional excretion of sodium). < 1% → prerenal. > 2% → intrinsic.
- Urine sediment. Muddy brown casts → ATN. RBC casts → GN. WBC casts + eosinophils → AIN. Hyaline only → prerenal.
Postrenal needs a renal ultrasound — hydronephrosis confirms it.
NBME-Style Stem
A 78-year-old man with metastatic prostate cancer presents with anuria for 24 hours. Serum creatinine has risen from 1.0 to 4.6 mg/dL over 3 days. Bladder is non-distended. Renal ultrasound shows bilateral hydronephrosis. Which of the following is the most likely cause of his acute kidney injury?
Concept Anchor
Three places blood and urine can fail in the kidney pipeline: not enough blood in (prerenal), the kidney itself is broken (intrinsic), or no urine out (postrenal). BUN:Cr and FENa split prerenal from intrinsic; ultrasound finds postrenal.