Renal

Acute kidney injury: staging, cause and what to do first

KDIGO staging, how to separate prerenal, intrinsic and postrenal causes with the bedside and urine findings, and the indications for dialysis.

10 min read·advanced·

Reviewed by a licensed nurse · 25 October 2026

KDIGO stages

Stage	Creatinine	Urine output
1	1.5 to 1.9 times baseline, or rise of 0.3 mg/dL or more	Under 0.5 mL/kg/h for 6 to 12 hours
2	2.0 to 2.9 times baseline	Under 0.5 mL/kg/h for 12 hours or more
3	3.0 times baseline, or 4.0 mg/dL or more, or dialysis started	Under 0.3 mL/kg/h for 24 hours or more, or no urine for 12 hours

Finding	Prerenal	Intrinsic (acute tubular necrosis)	Postrenal
BUN to creatinine ratio	Above 20	About 10 to 15	Variable
Fractional excretion of sodium	Under 1 percent	Above 2 percent	Variable
Urine sodium (mEq/L)	Under 20	Above 40	Variable
Urine sediment	Bland, hyaline casts	Muddy brown granular casts	Often bland. Hematuria possible.
Clue	Dehydration, bleeding, heart failure, sepsis	Prolonged hypotension, contrast, aminoglycosides	Enlarged prostate, stones, catheter blockage. Hydronephrosis on ultrasound.

Diuretics make FeNa unreliable. Use fractional excretion of urea instead (under 35 percent suggests prerenal).

01
Check for life-threatening problems
Hyperkalemia with ECG changes, severe acidosis, pulmonary edema.
02
Look for obstruction
Bladder scan, then a renal ultrasound. Relieving a blockage can restore function quickly.
03
Assess volume
Give balanced crystalloid to a hypovolemic patient and reassess. Do not give fluids to a patient who is already overloaded.
04
Remove the harm
Stop NSAIDs, ACE inhibitors, ARBs, contrast and other nephrotoxins where possible. Adjust the dose of renally cleared drugs.
05
Consider renal replacement
For refractory hyperkalemia, acidosis or volume overload, uremic pericarditis or encephalopathy, or dialyzable toxins.

Check yourself

1. A patient with vomiting and poor oral intake has creatinine 2.4 mg/dL (baseline 1.0), BUN 58 mg/dL, urine sodium 12 mEq/L and a bland sediment. What is the most likely cause?
2. Which finding is most suggestive of acute tubular necrosis?
3. Which indication is a reason to start dialysis?