The blood urea nitrogen (BUN)-creatinine ratio. Based on the information in Figure 12-3, the BUN-creatinine ratio often deviates from the usual value of about 10:1. These deviations may have modest diagnostic implications. As an example, for reasons as yet unclear, tubular reabsorption of urea nitrogen is enhanced in low-urine flow states. Thus, a high BUN-creatinine ratio often occurs in prerenal and postrenal (see Fig. 12-6) forms of renal failure. Similarly, enhanced delivery of amino acids to the liver (as with catabolism, corticosteroids, etc.) can enhance urea nitrogen formation and increase the BUN-creatinine ratio. A BUN-creatinine ratio lower than 10:1 can occur because of decreased urea nitrogen formation (eg, in protein malnutrition, advanced liver disease), enhanced creatinine formation (eg, with rhabdomyolysis), impaired tubular secretion of creatinine (eg, secondary to trimethoprim, cimetidine), or relatively enhanced removal of the small substance urea nitrogen by dialysis.
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