Differential Diagnosis for Elevated BUN with Normal Creatinine

Single Most Likely Diagnosis

• Dehydration: This is the most common cause of elevated Blood Urea Nitrogen (BUN) with normal creatinine. Dehydration leads to a decrease in blood volume, which in turn reduces renal blood flow and glomerular filtration rate, causing an increase in BUN levels. Since creatinine is less affected by dehydration, its levels remain normal.

Other Likely Diagnoses

• High protein diet: Consuming a diet high in protein can increase BUN levels because the body breaks down protein into urea, which is then excreted by the kidneys. If the kidneys are functioning normally (as indicated by normal creatinine), the creatinine level will not be affected.
• Gastrointestinal bleeding: Bleeding in the gastrointestinal tract can lead to an increase in BUN due to the absorption of blood proteins, which are then metabolized to urea. Creatinine levels are not directly affected by gastrointestinal bleeding.
• Heart failure: Congestive heart failure can cause a decrease in renal perfusion, leading to an increase in BUN. However, if the heart failure is not severe enough to cause significant renal dysfunction, creatinine levels may remain normal.

Do Not Miss Diagnoses

• Prerenal azotemia due to renal artery stenosis: Although less common, renal artery stenosis can cause a decrease in renal blood flow, leading to elevated BUN with relatively normal creatinine. Missing this diagnosis could lead to progressive renal damage.
• Early post-renal obstruction: Obstruction of the urinary tract can cause a build-up of urea, leading to elevated BUN. If the obstruction is partial or recent, creatinine levels might not be significantly affected.

Rare Diagnoses

• Corticosteroid use: Long-term use of corticosteroids can increase protein catabolism, leading to elevated BUN levels. Creatinine levels are generally not affected.
• Hyperthyroidism: Severe hyperthyroidism can increase protein catabolism and lead to elevated BUN. However, this is a rare cause and usually accompanied by other symptoms of hyperthyroidism.
• Rhabdomyolysis with minimal muscle involvement: Although rhabdomyolysis typically causes an increase in creatinine due to muscle breakdown, cases with minimal muscle involvement might present with elevated BUN and normal creatinine.