Renal Pathophysiology

CHAPTER 11 Acute Kidney Injury

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„ White cells and white cell casts—with or without some red cells—are highly suggestive of acute pyelonephritis or interstitial nephritis. „ A negative dipstick for protein or a normal microalbumin/creatinine ra tio with a positive urine protein/creatinine ratio points suggests myeloma kidney because the immunoglobulin light chains are not detected by the dipstick or the microalbumin assay. Prior to the widespread availability of urine protein/creatinine measurements, sulfosalicylic acid was used to precipitate nonalbumin proteins such as paraproteins. „ Many, often muddy-brown, granular casts with epithelial cells and epithe lial cell casts strongly suggest ATN; the urinary findings in this setting rep resent cellular debris and desquamation. „ A relatively normal urinalysis is typically seen in prerenal disease but may also occur in about 10% to 15% of cases of ATN and with urinary tract obstruction. Distinction Between Prerenal Disease and Acute Tubular Necrosis The different causes of prerenal disease and ATN are responsible for ∼ 75% of cases of AKI. Distinguishing between these disorders can be difficult because there is a continuum with the severity and duration of reduced renal perfusion in determining whether tubular damage has occurred. The major tool for distin guishing between prerenal disease and ATN is the response to intravenous fluids. An improvement in renal function (over 1 to 2 days) back to the baseline plasma creatinine concentration is considered diagnostic of prerenal disease, whereas a continued elevation in the plasma creatinine concentration points toward ATN. In addition, several other blood and urinary findings may be helpful (Table 11.2). These findings largely reflect the difference between intact tubu lar function in prerenal disease and impaired tubular function in ATN.

CHAPTER 11 Acute Kidney Injury

TABLE 11.2. Laboratory Findings in Prerenal Disease and ATN

Test

Favors Prerenal Disease

Favors ATN

> 20:1

10-15:1

BUN-to-PCr ratio

Many granular casts with renal tubular epithelial cells and epithelial cell casts

Urinalysis

Normal or near normal with few cells or casts; hyaline casts may be seen but are not abnormal.

< 25 mEq/L

> 40 mEq/L

Urine sodium

< 1%

> 2%

FENa

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> 500 mOsm/kg

300-350 mOsm/kg

Urine osmolality

ATN, acute tubular necrosis; BUN, blood urea nitrogen; PCr, plasma creatinine.