Which etiology results in an elevated bun level with a normal serum creatinine level?

Multiple causes. BUN is a breakdown of ingested protein, but becomes elevated compared to creatine, in cases of increased catabolism (breakdown). Such things as a high protein diet, use of some steroids, some infections, or volume depletion may commonly raise BUN levels. In a hospital setting, tube feeding, tpn, GI bleeding, infections, decreased flow to the kidneys, and use of steroids are frequent contributors.

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What results in an elevated bun level with a normal serum creatinine?

See below. An elevated BUN with normal creatinine occurs most commonly in dehydrated individuals and is not a sign of kidney damage.It may also occur in in certain other conditions like bleeding in the intestines, artificial nutrition intravenously of Amino Acids and use of steroids. Read more...
Many causes. Urea is the breakdown product of protein, and even a high-protein diet -- even eating a steak -- will raise it. Being dehydrated, as after a good workout, or (in those who are obviously sick) shock / GI bleeding / heart failure will raise BUN in preference to creatinine. And reference ranges are set so that several % of healthies fall outside. Labs mean nothing by themselves. Read more...