AKI — pre-renal, renal or post-renal
What it means
Pre-renal (commonest): hypovolaemia/hypotension/sepsis — the intact kidney avidly reabsorbs → urea rises out of proportion to creatinine, urine Na <20, FeNa <1%, corrects with fluids. Intrinsic/renal: ATN (prolonged pre-renal, contrast, rhabdomyolysis), glomerulonephritis (RBC casts, proteinuria), interstitial nephritis (drugs, eosinophils) — urine Na >40, FeNa >2%, muddy-brown casts. Post-renal: obstruction (stones, prostate, pelvic malignancy) — hydronephrosis on USS, palpable bladder; relieve the obstruction.
Worked example
Dehydration + urea raised disproportionately to creatinine + urine Na 12 that corrects with IV fluids → pre-renal AKI.
In the exam
A dehydrated, hypotensive patient has a urea raised out of proportion to the creatinine and a urine sodium of 12 mmol/L.
What settles it
Pre-renal = intact kidney conserving salt/water (low urine Na, high urea:creatinine, fluid-responsive); intrinsic = tubular/glomerular damage (high urine Na, casts).
Classically confused with
CK & rhabdomyolysis
Source: NICE NG148 (AKI); KDIGO