Urea – Creatinine Ratio

The relationship of urea and creatine is dependent on serum laboratory units used to determine the cause of acute kidney injury.

  • In the US the urea is expressed as BUN (Blood Urea Nitrogen) in mg/dL. Elsewhere Urea (U) is expressed as mmol/L
  • Similarly Creatinine (Cr) is expressed as mg/dL in the US and µmol/L elsewhere

Therefore two ratio’s exist to compare serum Urea and Creatinine levels

  • BUN : Cr ratio with US units of mg/dL : mg/dL
  • Urea: Cr ratio ith SI Units of mmol/L: µmol/L (providing Urea is >10mmol/L)
  • Cr: Urea ratio with SI Units of µmol/Lmmol/L (providing Urea is >10mmol/L)

Urea Creatinine ratio


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  • 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
  • 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 Nutr1992 Jul;15(1):89-92. PMID 1403455.
  • 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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  1. Rahul says

    Fantastic site. Was trying to understand the Ur -- Cr ratio and I think maybe the cloumns have been mixed up by mistake. Are my suspicions correct. Once again this is a wonderful tool and thanks for getting it up and running.

    • says

      Thanks Rahul
      The Urea:Creatinine ratio is tricky. I have changed the table to reflect U:Cr and Cr:U. I use the Cr:U ratio as I find it easier to remember the ratio as big to small.
      Hope this helps

  2. Martin Duffy says

    Thanks Mike for the quick reply regarding the Ur:Cr ratio.
    We were hoping it wasn’t that tricky! -- and maybe should have made our concerns a little clearer!
    The headings of the table were fine, it’s just that they were over the wrong columns -- BUN:Cr should have >20:1…etc, and Ur:Cr should have >100:1..etc (and you still get the large number first because the ratio is mmol/L:mmol/L as it should be).
    Love the site and we were just trying to help out.
    Martin Duffy -- on Rahul’s behalf!