According to KDIGO
1. Increase in serum creatinine > 0.3 mg/dL within 48 hours
2. Increase in serum creatinine > 1.5 times baseline within prior 7 days
3. Urine output < 0.5 mL/ kg/hr > 6 hrs (eg. 30cc /hr for 60kg ) 

FENA:  % urine sodium/urine creatinine ///plasma sodium/plasma creatinine X100 
FENa less than 1% prerenal :  Kidney current serving sodium due to hypoperfusion, dehydration, volume loss, CHF, cirrhosis
FENa greater than 2% intrinsic renal AKI, acute tubal necrosis, kidney can not reabsorbed sodium
FENA variable post renal cause or mixed status
FEUrea for diuretic use (increased urinary sodium, false elevation), CKD, nonoliguric AKI
Less than 35 % prerenal AKI
>50 % intrinsic renal AKI especially ATN
35-50% intermediate zone

Stage 1. 
Cr 1.5 to 1.9 baseline or 0.3 mg/dL, UO <0.5 less than 12 hours
Stage II
Cr 2.0 to 2.9, UO < 0.5 for 12 hours
Stage III
Cr 3.0, > 4 mL/dl, UO <0.3 for 24 hours, anuric for 12 hours