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Differential Diagnosis: Increased & Decreased Blood Urea Nitrogen

Julie Allen, BVMS, MS, MRCVS, DACVIM (SAIM), DACVP (Clinical), Durham, North Carolina

Urology & Nephrology

|July 2019|Peer Reviewed

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Following are differential diagnoses, listed in order of likelihood, for patients presented with increased or decreased blood urea nitrogen (BUN).

Increased BUN

Of note, increases in BUN should be interpreted in conjunction with urine specific gravity.

  • Dehydration
  • Renal disease
  • Postrenal obstruction or leakage
  • High-protein diet
  • GI bleeding
    • Reaction to drugs (eg, NSAIDs, high-dose/long-term steroid therapy)
  • Increased catabolism
    • Secondary to fever or cachexia
  • Breed predisposition (anecdotal)
    • Yorkshire terriers

Decreased BUN

  • Liver disease (due to decreased production)
    • Portosystemic shunting
    • Hepatic dysfunction
  • Polyuria/polydipsia (due to increased excretion; eg, Cushing’s disease, diabetes mellitus)
  • Low-protein diet
  • Increased anabolism or increased drinking and urination due to young age
  • Artifactually decreased BUN due to marked lipemia (lipemic index, >1000)
  • Anabolic steroids
  • Urea cycle enzyme deficiency


For global readers, a calculator to convert laboratory values, dosages, and other measurements to SI units can be found here.

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