Figure 518

Diagnostic approach to acute renal failure. Acute renal failure developing in a patient with malignancy may be due to diverse causes. It is important to employ an organized diagnostic approach to define the specific cause in a cost-effective manner. The approach outlined in this figure involves five steps. Step I addresses the distinction between acute and chronic renal failure, and step II lists the various causes of prerenal, intrinsic, and postrenal acute renal failure (see Figs. 5-2, 5-4, and 5-16) according to data obtained from the history and physical examination.

Urinalysis is very useful in the workup of a patient with acute renal failure, particularly due to intrinsic renal disease, as outlined in step III. The presence of red blood cell (RBC) casts or dysmorphic RBCs in the urine sedi-

ment is suggestive of glomerulonephritis, while eosinophiluria is indicative of acute interstitial nephritis. Step IV involves obtaining blood chemistries and other blood studies, abnormalities that may strongly support a given diagnosis. Step V is employed in the presence of oliguric acute renal failure. Urinary diagnostic indices are used to distinguish between prerenal acute renal failure and glomerulonephritis, as opposed to acute tubular necrosis or acute obstruction. Evaluation of the urine is also helpful in detecting the presence of light chains of immunoglobulins, which may be diagnostic of multiple myeloma-induced acute renal failure. Also, an increased urinary uric acid/creati-nine ratio may indicate acute uric acid nephropa-thy. In the patient who is anuric (<50 mL of urine per day), it is particularly important to rule out obstruction. Bilateral cortical necrosis or glomeru-lonephritis must be considered in this setting; a renal biopsy may be necessary for definitive diagnosis. If bilateral renal artery or vein occlusion is a consideration, angiography may be indicated. ATN—acute tubular necrosis; BUN— blood urea nitrogen; CHF—congestive heart failure; CPK—creatine phosphokinase; ECFV— extracellular fluid volume; FENa—fractional extraction of sodium; Hct—hematocrit; SPE— serum protein elec-trophoresis; Una—urine sodium; Uosm—urine osmolality; UPE—urine protein electrophoresis.

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