BUN-to-creatinine ratio

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Overview

In medicine, the BUN-to-creatinine ratio, also BUN-creatinine ratio, BUN/creatinine ratio or BUN:Cr, is the ratio of two serum laboratory values, the blood urea nitrogen (BUN) and serum creatinine. This terminology is used in the United States. In Canada and Europe, urea is used instead of BUN, so it is termed the urea-to-creatinine ratio, urea-creatinine ratio or urea/creatinine ratio. The interpretation and significance of the ratio is identical.

BUN:Cr Ratio Location of Renal disorder Mechanism
>20:1 Pre-Renal Disease (before glomeruli) Reduced flow causes elevated BUN reabsorption within kidney; Cr is not reabsorbed, therefore BUN:Cr ratio increases
15-20 Normal or Post-Renal Disease (within ureter) Normal range. In some instances, can be Post-Renal, as backflow from obstruction causes elevated BUN reabsorption within kidney; Cr is not reabsorbed, therefore BUN:Cr ratio increases
<15:1 Intrarenal Disease (within kidneys) Renal damage causes reduced BUN re-absorption, therefore lowering Bun:Cr ratio


Acute renal failure

It has been found to be predictive of pre-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.

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]

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.

Elderly patients

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

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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