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Learning Objectives

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  1. Recognize the association between the physiologic roles of the kidney and the laboratory assays used to assess renal function.

  2. Explain the basic concepts of the laboratory assays for renal function.

  3. Understand the diagnosis of specific renal disorders using clinical laboratory tests.

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Overview of Renal Disease

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The homeostatic roles of the kidney include maintenance and regulation of fluid balance, acid/base and electrolyte balance (eg, sodium, potassium, chloride, bicarbonate, calcium, phosphate, and magnesium), conservation of glucose, amino acids, and proteins, the excretion of wastes, and the production of hormones such as erythropoietin and 1,25-dihydroxyvitamin D. The renal blood vessels provide blood to the glomerulus and the tubules for the generation of urine. The glomerulus filters blood to create a plasma ultrafiltrate by retaining cells and proteins, whereas the tubules “process” the plasma ultrafiltrate to urine, thereby concentrating wastes such as urea, creatinine, nitrogenous wastes, and hydrogen ions.

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Renal disease is suggested by any of the following findings:

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  1. Nonspecific symptoms of malaise, headache, visual disturbances, nausea, or vomiting (eg, many of these findings suggest uremia or hypertension [see below]).

  2. Flank pain (eg, from pyelonephritis), pain that radiates to the groin from the flank (eg, from ureteral colic as a result of nephrolithiasis), or simple dysuria (eg, from a lower urinary tract infection).

  3. A reduction in the volume of urine output. In adults, oliguria, a pathologically reduced urine output, is defined as less than 500 mL of urine produced per day. Anuria, which is essentially absent urine production, is defined in adults as less than 100 mL of urine produced per day. In infants, oliguria can be defined as urine output of less than 1 mL/kg/h, and in children older than infants, oliguria is defined as urine output of less than 0.5 mL/kg/h.

  4. Hematuria, red blood cell casts, white blood cell casts, proteinuria, proteinaceous casts, pyuria, or other abnormalities on urinalysis.

  5. Discolored or malodorous urine (eg, from a urinary tract infection).

  6. Elevations in the plasma or serum concentrations of creatinine or blood urea nitrogen (BUN).

  7. Malar rash (eg, from systemic lupus erythematosus).

  8. Hypertension.

  9. Otherwise unexplained hypokalemia or hyperkalemia, hypocalcemia, hypophosphatemia or hyperphosphatemia, pathologic fractures, hypomagnesemia, acidosis, anemia, edema, or bleeding.

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Renal function should be evaluated when patients are taking drugs that can damage the kidney (eg, gentamicin) or drugs whose metabolism and/or excretion is dependent on the kidney (eg, low-molecular-weight heparin).

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Renal azotemia indicates that the kidney itself is dysfunctional. Renal azotemia results from diseases of the renal blood vessels, glomerulus, tubules, or mesangium.

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Nitrogen retention, as shown by an elevated BUN concentration, is termed “azotemia.” Azotemia can be classified as prerenal, renal, or postrenal. Prerenal azotemia refers to conditions with reduced blood flow to the kidney, thereby reducing urine output and causing the retention of waste products. Examples of prerenal causes of azotemia are congestive heart failure, GI hemorrhage, renal artery stenosis, ...

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