Acute Kidney Injury
An abrupt fall in kidney function classified by where the insult sits (before, within or beyond the kidney) because that anatomical logic dictates both the cause and the immediate treatment.
In a nutshell
AKI is sorted by anatomical compartment: pre-renal (perfusion), intrinsic (tissue), or post-renal (obstruction). Prolonged pre-renal injury converts to intrinsic tubular necrosis, nephrotoxins are dangerous because they remove the kidney's remaining compensations, and severity is staged by the trend in creatinine and urine output, not a single value.
Classic presentation
A hypotensive, hypovolaemic or septic patient with a rising creatinine and falling urine output, often with recent exposure to NSAIDs, ACE inhibitors/ARBs and diuretics together.
Key points
- Classify every case as pre-renal, intrinsic or post-renal before building a differential: the category dictates the treatment.
- Prolonged uncorrected pre-renal AKI becomes intrinsic acute tubular necrosis, which does not resolve simply by restoring perfusion.
- NSAIDs, ACE inhibitors/ARBs and diuretics together ('triple whammy') strip away the kidney's autoregulatory defences against hypoperfusion.
- KDIGO staging uses the trend in creatinine and urine output over time, not a single blood test.
- A palpable bladder or hydronephrosis on ultrasound means obstruction and needs urgent decompression, not fluids or dialysis first.
- Hyperkalaemia is the most immediately life-threatening complication and should be actively sought and treated.
First-line investigation
Serum U&E with creatinine trend against baseline, alongside urinalysis and a renal tract ultrasound to identify the anatomical compartment of injury.
First-line management
Correct the underlying cause: fluid resuscitation for hypoperfusion, stopping nephrotoxins, or relieving obstruction, while monitoring for and treating hyperkalaemia, acidosis and fluid overload.
Exam traps
- Do not give aggressive IV fluids to a patient with AKI and pulmonary oedema from fluid overload; assess volume status first.
- A normal single creatinine does not exclude AKI if the trend is rising or urine output is falling.
- Post-renal obstruction needs decompression, not diuretics or dialysis, as the first step.
- Stopping ACE inhibitors/ARBs in AKI is usually correct acutely, but do not assume every renally-acting drug should be stopped without considering the clinical context.
Educational content pending clinical review. Not medical advice.