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Differentiate prerenal azotemia from intrinsic kidney disease
Normal: 7-20 mg/dL
Normal: M 0.7-1.3 | F 0.6-1.1 mg/dL
| Finding | Prerenal | Intrinsic (ATN) | Postrenal |
|---|---|---|---|
| BUN/Cr Ratio | >20:1 | 10-15:1 | Variable |
| FENa | <1% | >2% | Variable |
| Urine Na | <20 mEq/L | >40 mEq/L | Variable |
| Urine Osmolality | >500 mOsm | <350 mOsm | Variable |
| Urine Sediment | Bland / hyaline | Muddy brown casts | Bland |
Acute tubular necrosis, glomerulonephritis - both rise proportionally or creatinine rises more.
Most useful in acute kidney injury to differentiate prerenal (reversible with fluids) from intrinsic renal disease (ATN). Should be combined with urinalysis, FENa, and clinical assessment.
Less reliable in: chronic kidney disease, patients on diuretics, contrast nephropathy, myoglobinuria, liver disease, or patients on steroids. Always interpret in clinical context.
Upper GI bleeding characteristically causes very high ratio (>30-40:1) because blood protein is absorbed and converted to urea. Lower GI bleeding has less effect on ratio.
High ratio (prerenal) = aggressive fluid resuscitation likely to help. Normal ratio with elevated values = less likely to respond to fluids alone, may need further workup.