Renal Failure - Pre-renal versus Acute Tubular Necrosis

  Pre-renal Renal
Pathophysiology Hypovolaemia
leading to intense tubular concentrating activity (retention of sodium and water)
( urine osmolality, [] of urea/creatinine) 
Tubular damage with
loss of tubular concentrating power
Plasma    

Plasma Urea:Creatinine 

> 100:1 <50:1
Spot urine    

Urine SG

> 1.015 < 1.010

Urine osm (mosm/L)

> 450 (> serum osm + 100) < 300 (> serum osm + 100)

Urine Na  (mmol/L)

< 20 > 40
Urine:Plasma     

UCr:PCr

> 40 <20

UUrea: PUrea

> 20 <10

UOsm:POsm

> 2:1 <1:2

Fe Na+

< 1% > 3%
Urine sediment    
  Normal Tubular casts/cells
Granular casts

Fe Na+ (Fractional excretion of Na)

     UNa x UCr
=   -----     -----
     PNa x PCr

= % of Na not reabsorped