Acute Renal Failure

Chapter 42 Acute Renal Failure









5 What are the implications of urinary electrolytes in the differential diagnosis of ARF?


The determination of urine electrolyte and creatinine concentrations may be helpful in the differential diagnosis of ARF. When used with serum values, urinary diagnostic indexes can be generated. Understanding the concepts behind the interpretation of these indexes is easier and better than trying to remember specific numbers. Quite simply, if the tubule is working well in the setting of decreased GFR, tubular reabsorption of sodium and water is avid, and the relative clearance of sodium to creatinine is low. Conversely, if the tubule is injured and cannot reabsorb sodium well, the relative clearance of sodium to creatinine is not low. Therefore, with prerenal azotemia, the ratio of the clearance of sodium to the clearance of creatinine, which is also called the fractional excretion of sodium (FENa) (FENa = [Urinary sodium]/[Urinary creatinine] × [Plasma creatinine]/[Plasma sodium] × 100), is typically less than 1.0, whereas with parenchymal or obstructive causes of ARF, the FENa is generally greater than 2.0 (Fig. 42-1).



The FENa test is much less useful when patients do not have oliguria. In this setting, the specificity of a low FENa for prerenal azotemia is markedly diminished. In addition to nonoliguria, several causes of ATN, specifically dye-induced ATN or ATN associated with hemolysis or rhabdomyolysis, may typically be associated with a low FENa. Patients who have prerenal azotemia but have either persistent diuretic effect, chronic tubulointerstitial injury, or bicarbonaturia may have a relatively high FENa. In the last case, the fractional excretion of chloride, which is calculated in an analogous way, will be appropriately low (< 1%). Finally, the early stages of ARF from glomerulonephritis, transplant allograft rejection, or urinary obstruction may be associated with a low FENa.



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Jul 6, 2016 | Posted by in CRITICAL CARE | Comments Off on Acute Renal Failure

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