BUN-to-creatinine ratio

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In medicine, the BUN-to-creatinine ratio, also BUN-creatinine ratio and BUN/creatinine ratio, is a ratio of two laboratory test values, the blood urea nitrogen (BUN) and serum creatinine. It is used in the United States; elsewhere (Canada, Europe) urea is used instead of BUN, so it is the urea-to-creatinine ratio, also urea-creatinine ratio and urea/creatinine ratio.

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[edit] Acute renal failure

It has been found to be predictive of pre-renal renal failure, if the BUN-to-creatinine ratio is greater than 20[1] or the urea-to-creatinine ratio>0.10 and urea>10.[2] In pre-renal failure, urea rises out of proportion to the creatinine due to enhanced proximal tubular reabsorption.

[edit] Gastrointestinal bleeds

It is useful for the diagnosis of upper gastrointestinal bleeding in patients who do not present with overt vomiting of blood.[3]

In children, a BUN/creatinine ratio of 30 or above has a sensitivity of 68.8% for upper GI bleeding and a specificity of 98%.[4]

[edit] Why the urea rises

The reason the urea concentration increases in upper GI bleeds is:

  • Blood, which consists largely of the protein hemoglobin, is broken down by digestive enzymes of the upper GI tract into amino acids.
  • The amino acids, which originate from the hemoglobin, are re-absorbed by the lower GI tract.
  • Urea is a break down product of amino acid catabolism; therefore, the "protein meal" from an upper GI bleed shows up in the blood as urea.

[edit] Elderly patients

Because of decreased muscle mass, elderly patients may have an elevated BUN-to-creatinine ratio at baseline.[5]

[edit] References

  1. ^ Morgan DB, Carver ME, Payne RB. Plasma creatinine and urea: creatinine ratio in patients with raised plasma urea. Br Med J. 1977 Oct 8;2(6092):929-32. PMID 912370.
  2. ^ Acute renal failure: urea:creatinine ratio was not very helpful in diagnosing prerenal failure. Evidence-Based On-Call database. URL: http://www.eboncall.org/CATs/1844.htm. Accessed on: June 25, 2006.
  3. ^ Witting MD, Magder L, Heins AE, Mattu A, Granja CA, Baumgarten M. ED predictors of upper gastrointestinal tract bleeding in patients without hematemesis. Am J Emerg Med. 2006 May;24(3):280-5. PMID 16635697.
  4. ^ Urashima M, Toyoda S, Nakano T, Matsuda S, Kobayashi N, Kitajima H, Tokushige A, Horita H, Akatsuka J, Maekawa K. BUN/Cr ratio as an index of gastrointestinal bleeding mass in children. J Pediatr Gastroenterol Nutr. 1992 Jul;15(1):89-92. PMID 1403455.
  5. ^ Feinfeld DA, Bargouthi H, Niaz Q, Carvounis CP. Massive and disproportionate elevation of blood urea nitrogen in acute azotemia. Int Urol Nephrol. 2002;34(1):143-5. PMID 12549657

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