Situation: A 54-year-old man weighing 60 kg is on post-operative day 2 after a bowel resection complicated by bleeding. On the day of surgery, his blood pressure fell to 82/50 mmHg for about 20 minutes until blood transfusion and intravenous fluids restored it. His creatinine before surgery was 1.0 mg/dL (88 µmol/L), and he has no history of kidney disease. He has received no contrast dye and no nephrotoxic drugs, he has an indwelling urinary catheter, and a bladder scan shows an empty bladder. Today his blood urea nitrogen (BUN) is 46 mg/dL (16.4 mmol/L), his creatinine is 1.8 mg/dL (159 µmol/L), and his urine output has averaged 22 mL/h over the last 14 hours. The team is managing him for acute kidney injury (AKI). His urine sodium is 12 mEq/L, his urine specific gravity is 1.028, and no casts are seen in the urine sediment. How should the nurse classify the most likely cause of his AKI?

His kidney function fell after a brief period of hypotension from bleeding, and his BUN-to-creatinine ratio is about 26:1 (46 ÷ 1.8), above 20:1, a pattern that suggests a prerenal cause. His low urine sodium, concentrated urine, and no muddy brown casts show that the tubules are still working, and there was no nephrotoxin or contrast; the bladder is empty and his creatinine was normal before surgery. Prerenal AKI is reversible when perfusion is restored but can progress to tubular damage if it is not corrected.

Step 1. Interpret the postoperative AKI picture

A 54-year-old man developed hypotension to 82/50 mmHg for about 20 minutes after bowel resection with bleeding. By postoperative day 2, his creatinine rose from a baseline of 1.0 mg/dL to 1.8 mg/dL, and urine output has averaged 22 mL/h over 14 hours. That is oliguria, and the acute rise in creatinine defines AKI. The key question is whether the injury is prerenal, intrarenal, or postrenal.

Step 2. Use urine indices to locate the cause

The urine sodium is 12 mEq/L, specific gravity is 1.028, and no casts are present. These findings point strongly to a prerenal mechanism. In prerenal AKI, the kidney is underperfused but the tubules are still intact and respond appropriately by retaining sodium and water. That produces a low urine sodium, a concentrated urine with high specific gravity, and a bland sediment without tubular casts. A urine sodium below 20 mEq/L and specific gravity above 1.020 are classic prerenal markers in oliguric states.

Step 3. Compare prerenal and intrarenal patterns

Intrarenal AKI from tubular damage usually follows prolonged or severe ischemia, sepsis, or nephrotoxin exposure. In that setting, damaged tubules lose the ability to concentrate urine and reabsorb sodium, so the urine sodium tends to be high, specific gravity is closer to isotonic, and muddy brown granular casts may appear. This patient had only a brief hypotensive episode, no nephrotoxin or contrast, and no casts, so tubular necrosis is less likely. Watch out! Prerenal AKI can progress to acute tubular necrosis if hypoperfusion is not corrected, but the current urine findings still indicate a prerenal phase.

Feature	Prerenal AKI	Intrarenal AKI (tubular)
Urine sodium	Low, often below 20 mEq/L	High, often above 40 mEq/L
Specific gravity	High, concentrated urine	Low or fixed, isotonic urine
Urine sediment	Bland, hyaline casts possible	Muddy brown granular casts
BUN-to-creatinine ratio	Often above 20:1	Usually below 20:1
Reversibility	Rapid if perfusion restored	Slower, may require dialysis

Step 4. Rule out postrenal and chronic causes

Postrenal AKI from obstruction is excluded because the indwelling catheter is draining and bladder scan shows an empty bladder. Chronic kidney disease is also unlikely because the preoperative creatinine was normal at 1.0 mg/dL and there is no history of kidney disease. The acute rise after surgery fits an acute process, not gradual nephron loss.

Step 5. Link the mechanism to the clinical course

The BUN is 46 mg/dL and creatinine is 1.8 mg/dL, giving a BUN-to-creatinine ratio of about 26:1. A ratio above 20:1 supports prerenal azotemia because urea reabsorption increases when tubular flow is slow, while creatinine continues to be filtered and excreted. The brief hemorrhagic hypotension reduced renal perfusion, and the kidney responded by conserving sodium and water. Key point! Prerenal AKI is reversible when perfusion is restored, but sustained hypoperfusion can convert it to intrarenal tubular injury.