Urine diagnostic indices (see Fig. 12-16) Consider need for further evaluation for obstruction
Ultrasonography, computed tomography, or magnetic resonance imaging Consider need for additional blood tests Vasculitis/glomerulopathy: human immunodeficiency virus infections, antineu-trophilic cytoplasmic antibodies, antinuclear antibodies, serologic tests for hepatitis, systemic bacterial endocarditis and streptococcal infections, rheumatoid factor, complement, cryoglobins Plasma cell disorders: urine for light chains, serum analysis for abnormal proteins Drug screen/level, additional chemical tests Consider need for evaluation of renal vascular supply
Isotope scans, Doppler sonography, angiography Consider need for more data to assess volume and cardiac status Swan-Ganz catheterization
Prerenal, postrenal, high oncotic pressure (dextran, mannitol)
Urinalysis in acute renal failure
RBC RBC casts Proteinuria
Glomerulopathy, vasculitis, thrombotic microangiopathy
WBC WBC casts iz
Pyelonephritis, interstitial nephritis
RTE cells Pigmented casts
Allergic interstitial nephritis, atheroemboli, glomerulopathy
ATN, myoglobinuria, hemoglobinuria iz
Uric acid, drugs or toxins
Low grade proteinuria
Plasma cell dyscrasia
Urinalysis in acute renal failure (ARF). A normal urinalysis suggests a prerenal or postrenal form of ARF; however, many patients with ARF of postrenal causes have some cellular elements on urinalysis. Relatively uncommon causes of ARF that usually present with oligoanuria and a normal urinalysis are mannitol toxicity and large doses of dextran infusion. In these disorders, a "hyperoncotic state" occurs in which glomerular capillary oncotic pressure, combined with the intratubular hydrostatic pressure, exceeds the glomerular capillary hydrostatic pressure and stop glomerular filtration. Red blood cells (RBCs) can be seen with all renal forms of ARF. When RBC casts are present, glomerulonephritis or vasculitis is most likely.
White blood cells (WBCs) can also be present in small numbers in the urine of patients with ARF. Large numbers of WBCs and WBC casts strongly suggest the presence of either pyelonephritis or acute interstitial nephritis. Eosinolphiluria (Hansel's stain) is often present in either allergic interstitial nephritis or atheroembolic disease [13, 14]. Renal tubular epithelial (RTE) cells and casts and pigmented granular casts typically are present in pigmenturia-associated ARF (see Fig. 12-21) and in established acute tubular necrosis (ATN). The presence of large numbers of crystals on urinalysis, in conjunction with the clinical history, may suggest uric acid, sulfonamides, or protease inhibitors as a cause of the renal failure.
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 . 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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