Definition
Any one of:
- Creatinine rise ≥ 26.5 µmol/L within 48 hours
- Creatinine rise to ≥ 1.5 × baseline within 7 days
- Urine output < 0.5 mL/kg/h for 6 hours
The three-way split
Pre-renal (the commonest) — hypoperfusion. Hypovolaemia, sepsis, heart failure, cirrhosis, renal artery stenosis, and the drug triad of ACE inhibitor/ARB + diuretic + NSAID.
Intrinsic renal — tubules, interstitium, glomeruli or vessels:
- Acute tubular necrosis: ischaemia or toxins (aminoglycosides, contrast, myoglobin, cisplatin)
- Acute interstitial nephritis: drugs (penicillins, PPIs, NSAIDs), classically with rash, fever and eosinophilia — though the full triad is uncommon
- Glomerulonephritis: red cell casts, dysmorphic red cells, proteinuria
- Vascular: thrombotic microangiopathy, cholesterol emboli after angiography
Post-renal — obstruction. Prostate, stones, malignancy, retroperitoneal fibrosis, a blocked catheter.
The two tests that sort it
Urinalysis and microscopy:
| Finding | Points to |
|---|---|
| Bland sediment, hyaline casts | Pre-renal |
| Muddy brown granular casts | Acute tubular necrosis |
| Red cell casts, dysmorphic RBCs | Glomerulonephritis |
| White cell casts, eosinophiluria | Interstitial nephritis |
| Crystals | Toxins, tumour lysis, some drugs |
Renal ultrasound to exclude obstruction — and it is the answer whenever the stem gives you an older man with a palpable bladder, or anuria.
Supporting indices: fractional excretion of sodium < 1% suggests pre-renal, > 2% suggests ATN. It is uninterpretable after diuretics — use fractional excretion of urea instead.
Management
- Treat the cause. Restore perfusion, relieve obstruction, stop the drug
- Stop nephrotoxins: NSAIDs, ACE inhibitors/ARBs, aminoglycosides, contrast where avoidable
- Dose-adjust everything else, including antibiotics and anticoagulants
- Fluids for hypovolaemia; be careful not to over-fill an oliguric patient
- Diuretics do not treat AKI — they treat fluid overload
Indications for urgent dialysis
Learn these as a list — they are asked directly:
- Acidosis, refractory
- Electrolytes — refractory hyperkalaemia
- Intoxication — dialysable poisons (lithium, salicylate, methanol, ethylene glycol, metformin)
- Overload — pulmonary oedema not responding to diuretics
- Uraemia — pericarditis, encephalopathy, bleeding
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