⚠️ Sometimes there can be more than one cause at the same time.
💡 Always check that any catheter is not blocked or that the tip is not sitting in the urethra.
📖 Definition
- Oliguria: < 500 ml urine output (UO) per day.
- Also defined as UO < 0.5 ml/kg/h for ≥ 6 consecutive hours (≈ 35 ml/hour for a 70 kg adult).
- Anuria: < 50 ml UO per day — usually suggests bilateral renal injury or obstruction of both kidneys.
📊 Incidence
- In ICU patients, nearly 50% experience at least one episode of oliguria during their stay.
🧬 Pathophysiology
- Average solute excretion = ~600 mOsm/day.
- Maximal urinary concentrating ability = 1200 mOsm/L → at least 0.5 L urine output is physiologically required.
- Urine output depends on:
- Glomerular filtration rate (GFR).
- Tubular secretion and reabsorption.
- Glomerular filtration is directly dependent on renal perfusion.
- Oliguria = either reduced GFR or mechanical obstruction to urine flow.
🩺 Causes (same classification as AKI)
- Pre-renal:
- Dehydration / hypovolaemia.
- Cardiac failure.
- Liver failure (hepatorenal syndrome).
- Renal (intrinsic):
- Nephrotoxic drugs (penicillin, NSAIDs, cephalosporins, gentamicin).
- Radiocontrast-induced nephropathy.
- Ischaemic acute tubular necrosis (ATN).
- Post-renal (obstructive):
- Renal pelvis: Papillary necrosis, tumour (often unilateral → oliguria not anuria).
- Ureter: Stone, compression, tumour (unilateral usually → oliguria).
- Bladder outlet: Transitional cell carcinoma, prostate cancer, BPH, blocked catheter, clot retention.
- Urethra: Stricture.
- Other: High intra-abdominal pressure.
🔎 Investigations
- Bloods: FBC, U&E, calcium, phosphate, LFTs, bone profile, glucose.
- Inflammatory markers: CRP (infection/inflammation, vasculitis).
- Imaging: CXR (oedema), AXR (abdominal pathology).
- Special tests: Serum protein electrophoresis.
- Urinalysis: Haematuria, leucocytes, protein, glucose.
- Haematuria + proteinuria → GN.
- Broad brown casts → ATN (ischaemic/toxic).
- RBC casts → acute glomerulonephritis.
- WCC casts → infection or interstitial nephritis.
- Urine sodium: Low (<10 mmol/L) in pre-renal.
- Urine osmolality: High (>500 mOsm/kg) in pre-renal.
- Autoimmune screen: ANA, dsDNA, ANCA, Anti-GBM if vasculitis suspected.
- Renal USS: To exclude obstruction/hydronephrosis (within 24h).
💊 Management
- 🔍 Check simple causes first: Blocked catheter or dehydration.
- 💧 Fluid challenge if volume depleted (unless pulmonary oedema present). Review response.
- 📈 Strict input/output chart; measure daily weight.
- 🩺 Assess BP and rule out shock — treat underlying cause, consider inotropes if needed.
- 💉 Central line may be required to guide fluid replacement and monitor response.
- 🖼️ Renal ultrasound within 24 hrs to rule out obstruction.
- ⛔ Stop nephrotoxic agents (NSAIDs, contrast, aminoglycosides).
- 📞 Early renal referral if oliguria/anuria persists despite optimisation, or if AKI worsens.
- 🔧 Obstructive causes → urology referral (may need stenting/nephrostomy).
📚 References
Clinical Pearl:
🚨 Always consider reversible causes first (blocked catheter, dehydration).
In exams, the key triad = Check catheter ➝ Assess volume ➝ Exclude obstruction.
