A nurse is caring for a client with diarrhea and dehydration. The nurse determines that client has received adequate fluid replacement if blood urea nitrogen decreases to:

Correct Answer: C. 18 mg/dL.

The normal value of blood urea nitrogen is 8 to 25 mg/dL. Fluid status absolutely affects the levels of BUN and creatinine in the blood, but volume depletion or dehydration tends to affect BUN more so that we see a BUN: creatinine ratio of 20:1 or more in people who are very dry.

• Option A: 36 mg/dl indicates a high level of BUN. Dehydration generally causes BUN levels to rise more than creatinine levels. This causes a high BUN-to-creatinine ratio. Kidney disease or blockage of the flow of urine from the kidney causes both BUN and creatinine levels to go up.
• Option B: 27 mg/dl still indicates dehydration. A patient who is severely dehydrated may also have a high BUN due to the lack of fluid volume to excrete waste products. Because urea is an end product of protein metabolism, a diet high in protein, such as high-protein tube feeding, may also cause the BUN to increase.
• Option D: A low BUN occurs with conditions such as fluid volume overload, malnutrition, etc. Because urea is synthesized by the liver, severe liver failure causes a reduction of urea in the blood. Just as dehydration may cause an elevated BUN, overhydration causes a decreased BUN. When a person has a “syndrome of inappropriate antidiuretic secretion” (SIADH), the antidiuretic hormone responsible for stimulating the kidney to conserve water causes excess water to be retained in the bloodstream rather than being excreted into the urine.