KDIGO Definition
AKI is defined as a creatinine rise of 26.5 micromol/L (0.3 mg/dL) or more within 48 hours, a rise to 1.5 times baseline within 7 days, or urine output below 0.5 mL/kg/h for 6 hours.
The Three Categories
- Pre-renal — reduced renal perfusion, tubules intact (hypovolemia, hypotension, heart failure, renal artery stenosis)
- Intrinsic (renal) — damage within the kidney itself; acute tubular necrosis (ATN) is the most common cause, also glomerulonephritis, interstitial nephritis, vasculitis
- Post-renal — obstruction to urine flow (stones, BPH, tumor, blocked catheter)
Labs That Help Differentiate
| Test | Pre-renal | Intrinsic (ATN) |
|---|---|---|
| FENa | <1% | >2% |
| Urine sodium | <20 mEq/L | >40 mEq/L |
| BUN:Creatinine ratio | >20:1 | <15:1 |
| Urinalysis | Normal / hyaline casts | Muddy brown casts, tubular cells |
Initial Workup
Urinalysis, renal ultrasound (rule out obstruction first — it's reversible and easy to miss), a full medication review to identify and stop nephrotoxins, volume status assessment, and FENa where indicated.