Urea-Creatinine Ratio

OVERVIEW

Principle

  • urea and creatinine both freely filtered at the glomerulus
  • creatinine is not reabsorbed
  • urea reabsorbed by tubules via regulation
  • can be used as an indicator of the likely cause of renal failure

Normal Urea – 2.5-10.7mmol/L
Normal Creatinine – 62-106umol/L -> 0.062 – 0.106mmol/L (divide micromoles by 1000)

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)

CAUSES

Urea:Creatinine Ratio (in the setting of renal failure / elevated creatinine)

  • 40-100:1 – normal or post renal cause of AKI
  • >100:1 – pre-renal cause (urea absorption increased compared to creatinine)
  • <40:1 – intrinsic renal damage (urea unable to be absorbed -> become like creatinine -> ratio gets closer to 1)

Other causes:

INCREASED UREA:CREATININE RATIO – (Drivers Can use GPS)

  • dehydration/prerenal failure
  • corticosteroids
  • GI haemorrhage
  • protein-rich diet
  • severe catabolic state

DECREASED UREA:CREATININE RATIO (I am a SIMPLE SR)

  • severe liver dysfunction
  • intrinsic renal damage
  • malnutrition
  • pregnancy
  • low protein diet
  • SIADH
  • rhabdomyolysis

References

  • 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
  • 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

CCC 700 6

Critical Care

Compendium

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