Comparison Table
| Feature |
Prerenal |
Intrinsic |
Postrenal |
| Pathophysiology |
Hypoperfusion (intact parenchyma) |
Direct damage to parenchyma (ATN, AIN) |
Obstruction of outflow |
| BUN/Cr |
> 20:1 |
< 15:1 |
Variable (> 20:1 early, < 15:1 late) |
| FENa |
< 1% |
> 2% |
> 2% (late) |
| Urine Osm |
> 500 |
< 350 |
< 350 |
| Urine Na |
< 20 |
> 40 |
> 40 |
| Sediment |
Hyaline Casts |
Granular/Muddy Brown, RBC, WBC Casts |
Bland / Variable |
Key Formulas
FENa = (PCr × UNa) / (PNa × UCr) × 100
Measures % of filtered Na excreted. < 1% = kidney saving salt (Prerenal). > 2% = kidney losing salt (Intrinsic).
Urea Logic
Urea follows water. In Prerenal, ADH reabsorbs water AND Urea -> High BUN. Cr is not reabsorbed.
Mnemonics
- "Brown" Casts: ATN (Dead cells = mud).
- "FeNa < 1 (ONE)": Only Needs Electrolytes (Body is thirsty, holds Na).
- Postrenal = "Plumbing": Blocked pipe backs up pressure.