Acute Kidney Injury
Rapid decline in kidney function over hours to days, defined by KDIGO criteria as a rise in serum creatinine of ≥26.5 µmol/L within 48 hours or ≥1.5× baseline within 7 days, or urine output <0.5 mL/kg/hr for 6 hours. Common causes include sepsis, hypovolaemia, nephrotoxins, and obstruction.
Key Facts
AKI affects 13-18% of hospitalised patients and carries an in-hospital mortality of 20-30% in severe cases KDIGO staging: Stage 1 (creatinine 1.5-1.9× baseline), Stage 2 (2.0-2.9×), Stage 3 (≥3.0× or ≥353.6 µmol/L or RRT) Classified as pre-renal (55-60%), intrinsic renal (35-40%), or post-renal/obstructive (5-10%) NICE NG148 recommends risk assessment using NEWS2 score and detection via NHS AKI e-alert system First-line investigation: urinalysis, renal USS within 24 hours if obstruction suspected, and U&Es with serial monitoring Management: IV fluid resuscitation (crystalloid, typically 0.9% NaCl), stop nephrotoxins (NSAIDs, ACEi, ARBs, aminoglycosides), treat underlying cause Indications for urgent dialysis: refractory hyperkalaemia, severe metabolic acidosis (pH <7.1), pulmonary oedema, uraemic pericarditis/encephalopathy 50% of AKI is preventable with early recognition and intervention
Overview
Key Facts
AKI is a common and serious condition with significant morbidity and mortality. Early detection via the NHS AKI e-alert system and prompt management substantially improve outcomes. Prevention is key, particularly in high-risk groups.
Epidemiology
- Affects 13-18% of hospital admissions in the UK
- Community-acquired AKI accounts for ~50% of cases
- In-hospital mortality: 20-30% for severe AKI (stage 3); overall mortality 10-15%
- ICU incidence: up to 50% of critically ill patients develop AKI
- Survivors have increased long-term risk of CKD progression
Aetiology
Pre-renal (55-60%):
- Hypovolaemia: haemorrhage, dehydration, burns, diarrhoea/vomiting
- Reduced cardiac output: heart failure, cardiogenic shock
- Systemic vasodilation: sepsis, anaphylaxis, liver failure
- Renal vasoconstriction: NSAIDs (block afferent arteriolar dilation), ACEi/ARBs (block efferent arteriolar constriction), hepatorenal syndrome
Intrinsic renal (35-40%):
- Acute tubular necrosis (ATN): ischaemic (prolonged pre-renal) or nephrotoxic (aminoglycosides, contrast, cisplatin)
- Acute interstitial nephritis: drugs (NSAIDs, PPIs, penicillins), autoimmune
- Glomerulonephritis: RPGN, anti-GBM, lupus nephritis
- Vascular: renal artery stenosis/thrombosis, HUS/TTP
Post-renal/Obstructive (5-10%):
- Prostatic hypertrophy, prostate cancer
- Renal/ureteric calculi
- Pelvic malignancy, retroperitoneal fibrosis
Pathophysiology
- Pre-renal: reduced renal perfusion leads to decreased GFR; reversible if corrected early, but prolonged hypoperfusion causes ATN
- ATN: ischaemic or toxic injury to tubular epithelial cells; characterised by muddy brown granular casts
- Post-renal: bilateral obstruction (or unilateral in single kidney) causes back-pressure and reduced GFR
Clinical Presentation
General Presentation
- Oliguria (<0.5 mL/kg/hr) or anuria
- Fluid overload: peripheral oedema, pulmonary oedema, raised JVP
- Nausea, vomiting, anorexia
- Lethargy, confusion (uraemic encephalopathy)
- Pruritus and uraemic frost (severe)
Pre-renal Features
- Thirst, postural hypotension, tachycardia, dry mucous membranes
- Low urine sodium (<20 mmol/L), high urine osmolality (>500 mOsm/kg)
- Responds to fluid resuscitation
Intrinsic Renal Features
- Haematuria, proteinuria (glomerulonephritis)
- Rash, fever, eosinophilia (interstitial nephritis)
- Muddy brown casts on urinalysis (ATN)
Post-renal Features
- Suprapubic fullness, palpable bladder
- Loin pain, anuria alternating with polyuria
- History of prostatic symptoms or pelvic malignancy
Red Flags
- Pulmonary oedema unresponsive to diuretics → urgent dialysis
- K+ >6.5 mmol/L with ECG changes → emergency treatment
- Uraemic pericarditis (friction rub) → dialysis indication
- Rapidly rising creatinine with active sediment → consider RPGN (renal biopsy urgently)
Differential Diagnosis
| Diagnosis | Key Features | Investigation |
|---|---|---|
| CKD | Small kidneys on USS, normocytic anaemia, raised PTH | Renal USS, previous creatinine trend |
| AKI on CKD | Acute deterioration on chronic baseline | Previous blood results, renal USS |
| Hepatorenal syndrome | Cirrhosis, ascites, progressive renal failure | Hepatic function, urine sodium <10 |
| Urinary obstruction | Bilateral hydronephrosis, bladder distension | Renal USS, bladder scan |
| Rhabdomyolysis | Muscle pain, dark urine, massively raised CK | CK, urine myoglobin |
| Glomerulonephritis | Active urine sediment, systemic features | Urine protein:creatinine, immunology |
Diagnosis / Investigation
Bedside
- Urinalysis: dipstick for blood/protein (active sediment suggests GN); leucocytes/nitrites (infection)
- Bladder scan: post-void residual (exclude urinary retention)
- Fluid balance chart: strict input/output monitoring
- ECG: assess for hyperkalaemia changes
- Blood gas: rapid potassium, pH, bicarbonate, lactate
Bloods
- U&Es: creatinine, urea, potassium, sodium – serial monitoring
- FBC: anaemia (may suggest chronicity), thrombocytopenia (HUS/TTP)
- CRP: infection/sepsis
- LFTs: hepatorenal syndrome
- Calcium, phosphate, bone profile: deranged in established renal failure
- CK: rhabdomyolysis
- Blood cultures: if sepsis suspected
- Immunology (if intrinsic renal cause suspected): ANA, ANCA, anti-GBM, complement (C3/C4), immunoglobulins
Imaging
- Renal ultrasound: within 24 hours if obstruction suspected; assess kidney size (small = chronic), hydronephrosis
- CT KUB: renal stones if suspected
- CT abdomen/pelvis: retroperitoneal pathology, pelvic malignancy
Special Tests
- Urine sodium and osmolality: pre-renal (<20 mmol/L, >500 mOsm/kg) vs ATN (>40 mmol/L, <350 mOsm/kg)
- Fractional excretion of sodium (FENa): <1% pre-renal, >2% ATN
- Urine protein:creatinine ratio (uPCR): quantify proteinuria
- Renal biopsy: if intrinsic cause unclear, RPGN suspected, or unexplained AKI with active sediment
Management
Non-pharmacological
- Fluid resuscitation: isotonic crystalloid (0.9% NaCl or Hartmann's) for pre-renal AKI; 250-500mL boluses with reassessment
- Stop nephrotoxins: NSAIDs, ACE inhibitors, ARBs, aminoglycosides, metformin
- Relieve obstruction: urinary catheter (urinary retention), nephrostomy or ureteric stent (ureteric obstruction)
- Strict fluid balance and daily weights
- Dietary modification: potassium and phosphate restriction if required
Pharmacological
- Treat underlying cause: antibiotics for sepsis, immunosuppression for GN
- IV fluids: guided by clinical assessment and fluid balance
- Loop diuretics (furosemide 40-250mg IV): only for fluid overload, NOT to prevent or treat AKI itself
- Hyperkalaemia management: insulin-dextrose, calcium gluconate, salbutamol as per protocol
- Sodium bicarbonate 1.26%: for severe metabolic acidosis (pH <7.2)
- Avoid contrast media; if essential, use lowest volume with pre-hydration (0.9% NaCl 1mL/kg/hr for 12 hours pre and post)
Renal Replacement Therapy (Indications)
Urgent dialysis indicated for refractory:
- Hyperkalaemia (K+ >6.5 despite medical management)
- Acidosis (pH <7.1 despite bicarbonate)
- Pulmonary oedema (unresponsive to diuretics)
- Uraemic complications: encephalopathy, pericarditis, neuropathy
- Toxins: methanol, ethylene glycol, lithium, salicylate
Modes: CVVH/CVVHDF (ICU), intermittent haemodialysis (stable patients)
Referral Criteria
- AKI stage 3 or requiring RRT → nephrology
- Suspected GN or vasculitis → urgent nephrology
- Obstruction requiring intervention → urology
- ICU admission if haemodynamically unstable or requiring organ support
Prognosis
- Overall in-hospital mortality: 10-15% for all AKI; 30-50% in ICU
- Stage 1 AKI: mortality ~5-10%
- Stage 3 AKI requiring dialysis: mortality 30-50%
- Recovery: 50-70% of patients with AKI have full renal recovery
- 20-50% of AKI survivors develop or progress to CKD within 5 years
- 5-10% of severe AKI patients require long-term dialysis
- AKI increases risk of future cardiovascular events by 40%
- Prevention of a single AKI episode saves the NHS approximately £12,000-£25,000
Other Relevant Information
KDIGO AKI Staging
| Stage | Serum Creatinine | Urine Output |
|---|---|---|
| 1 | 1.5-1.9× baseline or ≥26.5 µmol/L rise in 48h | <0.5 mL/kg/h for 6-12h |
| 2 | 2.0-2.9× baseline | <0.5 mL/kg/h for ≥12h |
| 3 | ≥3.0× baseline or ≥353.6 µmol/L or RRT initiated | <0.3 mL/kg/h for ≥24h or anuria ≥12h |
Pre-renal vs ATN
| Feature | Pre-renal | ATN |
|---|---|---|
| Urine Na+ | <20 mmol/L | >40 mmol/L |
| FENa | <1% | >2% |
| Urine osmolality | >500 mOsm/kg | <350 mOsm/kg |
| Urine:plasma urea | >10:1 | <10:1 |
| Urine sediment | Bland/hyaline casts | Muddy brown casts |
| Response to fluids | Improves | No improvement |
Indications for Urgent Dialysis (Mnemonic: AEIOU)
- Acidosis (pH <7.1)
- Electrolytes (refractory hyperkalaemia)
- Intoxication (methanol, ethylene glycol, lithium)
- Overload (pulmonary oedema refractory to diuretics)
- Uraemia (encephalopathy, pericarditis)