Synopsis
Recognise acute kidney injury promptly, apply creatinine and urine-output staging correctly, identify immediate threats and start a cause-directed assessment before reversible injury progresses.
- Diagnose AKI in an adult when serum creatinine rises by at least 26 micromol/L within 48 hours, rises to at least 1.5 times a known or presumed baseline within 7 days, or urine output falls below 0.5 mL/kg/hour for more than 6 hours.
- Use the worst KDIGO creatinine or urine-output criterion. Stage 1: creatinine 1.5–1.9 times baseline or a rise of at least 26 micromol/L; urine output below 0.5 mL/kg/hour for 6–12 hours. Stage 2: creatinine 2.0–2.9 times baseline; urine output below 0.5 mL/kg/hour for at least 12 hours. Stage 3: creatinine at least 3 times baseline, an increase to at least 354 micromol/L with an acute rise of at least 44 micromol/L, or initiation of renal replacement therapy; urine output below 0.3 mL/kg/hour for at least 24 hours or anuria for at least 12 hours.
- Creatinine is a delayed concentration marker influenced by muscle mass, fluid balance and laboratory timing; an apparently normal result cannot exclude evolving injury after shock, sepsis or toxin exposure.
- Retrieve previous results and reconstruct the timeline. The same creatinine may represent stable CKD, AKI on CKD, or a major acute fall in filtration in a person with low muscle mass.
- Measure and document urine output rather than relying on the phrase ‘passing urine’; non-oliguric AKI can still be severe, while blocked catheters and inaccurate fluid charts can mimic oliguria.
- Classify the likely mechanism as impaired perfusion, intrinsic renal injury or urinary obstruction, while recognising that hospital AKI commonly has several concurrent causes.
- Review every medicine for haemodynamic effect, direct nephrotoxicity, renal accumulation and the consequences of temporary interruption; create an explicit restart plan rather than allowing important long-term therapy to disappear.
- Escalation follows physiology and trajectory, not a creatinine threshold alone: rapidly rising potassium, oxygen requirement, acidosis, altered cognition or worsening oliguria may demand action before stage 3 is reached.
Key red flags
A rise of at least 26 micromol/L within 48 hours or at least 50% above baseline within 7 days meets an adult AKI criterion. Search laboratory records for a credible baseline and state whether the timing is known, estimated or uncertain.
Output below 0.5 mL/kg/hour for over 6 hours is sufficient even before creatinine rises. Confirm measurement, catheter patency and body weight, and distinguish oliguria from complete anuria because anuria strongly raises obstruction or catastrophic vascular disease.
Blood and protein on urinalysis, red-cell casts if microscopy is available, hypertension, oedema, rash, pulmonary haemorrhage or systemic vasculitic features suggest glomerular or interstitial disease and warrant early nephrology discussion rather than repeated fluid challenges.
ECG change from hyperkalaemia, refractory pulmonary oedema, severe metabolic acidosis, uraemic pericarditic pain or encephalopathy, and a suspected infected obstructed system are time-critical complications whose treatment must begin while staging and cause are refined.
Investigation priorities
Confirm an acute change, stage severity and expose dangerous potassium or bicarbonate disturbance.
Management branches
A laboratory alert, new oliguria or clinical suspicion of acute renal dysfunction.
- 1. Perform ABCDE, obtain observations, assess perfusion and congestion, verify urine output and check the catheter or bladder when relevant.
- 2. Send urgent creatinine, electrolytes and venous gas, obtain an ECG if potassium may be raised, and begin treatment of any immediate physiological threat.
- 3. Retrieve baseline results, apply both creatinine and urine-output criteria, record the highest stage and document the direction and speed of change.
- 4. Review medicines and exposures, treat sepsis or shock promptly, and call renal, critical care or urology early when complications or diagnostic red flags are present.