⚠️ Must-Not-Miss Diagnoses
Obstructive uropathy (treatable with catheter or stent), rapidly progressive GN (requires urgent kidney biopsy and immunosuppression), renal artery occlusion, acute cortical necrosis.
Differential Diagnosis by Key Features
| Clinical Feature | Think Of | Priority |
|---|---|---|
| Volume depletion: vomiting, diarrhea, bleeding, poor intake | Prerenal AKI | Common — fluids first |
| NSAIDs, ACE inhibitors, contrast, aminoglycosides, vancomycin | Nephrotoxic ATN | Common — stop offending agent |
| Inability to void, suprapubic fullness, BPH, pelvic malignancy | Obstructive uropathy | HIGH — Foley catheter immediately |
| Hematuria + RBC casts + proteinuria + hypertension | Glomerulonephritis | HIGH — nephrology urgently |
| Recent contrast or NSAID, no casts, fractional excretion low | Contrast nephropathy / prerenal | Moderate |
| Muddy brown granular casts, prior hypotension or nephrotoxin | ATN | Moderate |
| Rash + eosinophilia + recent new medication | Acute interstitial nephritis | Moderate — stop drug |
Systematic Approach
Exclude Obstruction
Bladder ultrasound immediately
Postrenal AKI is the most immediately reversible cause. Bedside bladder ultrasound takes 60 seconds and rules out urinary retention. Renal ultrasound evaluates for hydronephrosis from upper tract obstruction.
Assess Volume
Clinical exam + response to fluids
Examine for signs of hypovolemia (dry mucous membranes, poor skin turgor, orthostatic hypotension) vs euvolemia vs hypervolemia. Judicious fluid challenge helps distinguish prerenal from intrinsic in unclear cases.
Urine Studies
FENa, urine microscopy, urine protein
FENa <1%: prerenal (kidneys retaining sodium). FENa >2%: ATN (tubules can't retain sodium). Exception: contrast nephropathy and early obstruction may show low FENa. Granular muddy brown casts = ATN. RBC casts = GN.
Medication Review
Remove nephrotoxins immediately
Review all medications: NSAIDs, ACE inhibitors/ARBs (reduce GFR in low-flow states), aminoglycosides, vancomycin, contrast agents, calcineurin inhibitors. Stopping the offending agent is often the most important intervention.
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