- 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/L: mmol/L (providing Urea is >10mmol/L)
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)
INCREASED UREA:CREATININE RATIO – (Drivers Can use GPS)
- dehydration/prerenal failure
- GI haemorrhage
- protein-rich diet
- severe catabolic state
DECREASED UREA:CREATININE RATIO (I am a SIMPLE SR)
- severe liver dysfunction
- intrinsic renal damage
- low protein diet
- 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
Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.
After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.
He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE. He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.
His one great achievement is being the father of three amazing children.
On Twitter, he is @precordialthump.