Lab Findings in Prerenal Azotemia and ARF
With hypoperfusion, the urine is typically concentrated with an osmalilty >500 mOsm/kg and a high specific gravity >1.020.
In ATN, concentrating ability is lost and the urine is usually isomolar to the serum. In the blood, the ratio of BUN to creatinine is normally <20:1 and an elevation is consistent with hypovolemia.
BUN/Cr. An ▲ BUN relative to Cr may also be due to increased urea production that occurs with hyperalimentation (high-protein), glucocorticoid therapy, and GIB.
FeNa has limited utility when ARF is superimposed on CKD.
Serologic testing can further delineate the cause of ARF
Antiglomerular basement membrane, Ab in Goodpasure's syndrome.
ANCA-ab (C-ANCA) in microscopic PAN or Wegener's syndrome
ANA in SLE