Acute Kidney Injury
Acute Kidney Injury (AKI) questions test KDIGO criteria, the prerenal–intrinsic–postrenal approach, urine microscopy and renal replacement timing.
Core rules
- KDIGO: SCr rise ≥26.5 µmol/L in 48 h, or ≥1.5× baseline within 7 days, or urine output <0.5 mL/kg/h for 6 h.
- Stage 1: 1.5–1.9× or ≥26.5; stage 2: 2.0–2.9×; stage 3: ≥3×, SCr ≥353.6 µmol/L, or RRT.
- Prerenal: bland urine, FENa <1% (unreliable with diuretics/CKD); ATN: muddy-brown granular casts, FENa >2%; glomerulonephritis: dysmorphic RBCs/RBC casts; pyelonephritis: WBC casts.
- RRT indications: refractory acidosis, hyperkalaemia, intoxication, fluid overload, uraemic complications.
Worked example
A 70-kg man with baseline creatinine 90 µmol/L reaches 210 µmol/L on day 3. Ratio 210/90 = 2.33, so KDIGO stage 2. Urine output 0.4 mL/kg/h for 8 h meets stage 1 output criterion only; creatinine stage dominates. He is not oliguric by the 12-h stage 2 threshold.
Traps
- Using the first ICU creatinine as baseline overstates AKI; use the lowest recent value.
- FENa is uninterpretable during diuresis; fractional urea excretion or FEK may help.
- Contrast is rarely the sole cause; look for sepsis, hypovolaemia and nephrotoxins.
- Creatinine lags injury and is diluted by fluid overload.