BUN-to-Creatinine Ratio

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Subject: BUN-to-Creatinine Ratio

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Definition and Use

  • The BUN-to-creatinine ratio is used to differentiate prerenal and postrenal azotemia from renal azotemia. Because of considerable variability, it should be used only as a rough guide.

  • Normal range (usual range for most people on normal diet: 12–16).

Interpretation

Increased Ratio (>10:1) with Normal Creatinine In

  • Prerenal azotemia (e.g., heart failure, salt depletion, dehydration, blood loss) due to decreased GFR

  • Catabolic states with increased tissue breakdown

  • GI hemorrhage; a ratio ≥36 is reported to distinguish upper from lower GI hemorrhage in patients with negative gastric aspirate.

  • High protein intake

  • Impaired renal function plus

    • Excess protein intake or production or tissue breakdown (e.g., GI bleeding, thyrotoxicosis, infection, Cushing syndrome, high-protein diet, surgery, burns, cachexia, high fever)

    • Urine reabsorption (e.g., ureterocolostomy)

    • Patients with reduced muscle mass (subnormal creatinine production)

  • Certain drugs (e.g., tetracycline, glucocorticoids)

  • Selective increase in plasma urea (diuretic-induced azotemia) during use of loop diuretics

Increased Ratio (>10:1) with Elevated Creatinine In

  • Postrenal azotemia (BUN rises disproportionately more than creatinine) (e.g., obstructive uropathy)

  • Prerenal azotemia superimposed on renal disease

Decreased Ratio (<10:1) with Decreased BUN In

  • Acute tubular necrosis

  • Low-protein diet, starvation, severe liver disease, and other causes of decreased urea synthesis

  • Repeated dialysis (urea rather than creatinine diffuses out of extracellular fluid)

  • Inherited deficiency of urea cycle enzymes (e.g., hyperammonemias—urea is virtually absent in blood)

  • SIADH (due to tubular secretion of urea)

  • Pregnancy

Decreased Ratio (<10:1) with Increased Creatinine In

  • Phenacemide therapy (accelerates conversion of creatine to creatinine)

  • Rhabdomyolysis (releases muscle creatinine)

  • Muscular patients who develop renal failure

Limitations

  • DKA (acetoacetate causes false increase in creatinine with certain methodologies, resulting in normal or decreased ratio when dehydration should produce an increased ratio)

  • Cephalosporin therapy (interferes with creatinine measurement)

 
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