Figure 1216

Urinary diagnostic indices in acute renal failure (ARF). These indices have traditionally been used in the setting of oliguria, to help differentiate between prerenal (intact tubular function) and acute tubular necrosis (ATN, impaired tubular function). Several caveats to interpretation of these indices are in order [1]. First, none of these is completely sensitive or specific in differentiating the prer-enal from the ATN form of ARF. Second, often a continuum exists between early prerenal conditions and late prerenal conditions that lead to ischemic ATN. Most of the data depicted here are derived from patients relatively late in the progress of ARF when the serum creatinine concentrations were 3 to 5 mg/dL. Third, there is often a relatively large "gray area," in which the various indices do not give definitive results. Finally, some of the indices (eg, fractional excretion of endogenous lithium [FE lithium]) are not readily available in the clinical setting. The fractional excretion (FE) of a substance is determined by the formula: U/P substance h- U/P creatinine X 100. U/P—urine-plasma ratio.

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