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

Conversion: SI to traditional units

• Plasma urea: 10 mmol/l = 27.8 mg/100 ml
• Plasma creatinine: 100 µmol/ – 1.13 mg/100 ml.

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

• BUN : Cr ratio with US units of mg/dL : mg/dL – this is most commonly used in online calculators and converts the SI units to US units e.g. MDcalc
• Urea : Cr ratio with SI Units of mmol/L: mmol/L (providing Urea is >10mmol/L) – [note to calculate the Urea : Cr ratio it is required to convert the Creatinine from µmol/L to mmol/L] [hat tip Dr Julian Pecora]
• Cr : Urea ratio with SI Units of µmol/L: mmol/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
• 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 Differential Diagnosis Series
NEURO	Anosmia, Ataxia, Blepharospasm, Bulbar and Pseudobulbar palsy, Central Pontine Myelinosis, Cerebellar Disease, Chorea, Cranial nerve lesions, Dementia, Dystonia, Exophthalmos, Eye trauma, Facial twitches, Fixed dilated pupil, Horner syndrome, Loss of vision, Meningism, Movement disorders, Optic disc abnormality, Parkinsonism, Peripheral neuropathy, Radiculopathy, Red eye, Retinal Haemorrhage, Seizures, Sudden severe headache, Tremor, Tunnel vision
RESP	Bronchial breath sounds, Bronchiectasis, High airway pressures, Massive haemoptysis, Sore throat, Tracheal displacement
CVS	Atrial Fibrillation, Bradycardia, Cardiac Failure, Chest Pain, Murmurs, Post-resuscitation syndrome, Pulseless Electrical Activity (PEA), Pulsus Paradoxus, Shock, Supraventricular tachycardia (SVT), Tachycardia, VT and VF, SVC Obstruction
GIT	Abdominal distension, Abdominal mass, Abdominal pain, Asterixis, Dysphagia, Hepatomegaly, Hepatosplenomegaly, Large bowel obstruction, Liver palpation abnormalities, Lower GI haemorrhage, Malabsorption, Medical causes of abdominal pain, Rectal mass, Small bowel obstruction, Upper GI Haemorrhage
GUT	Genital ulcers, Groin lump, Scrotal mass, Urine colour, Urine Odour, Urine transparency,
MSK	Arthritis, Shoulder pain, Wasting of the small muscles of the hand
DERM	Palmar erythema, Serious skin signs in sick patients, Thickened Tethered Skin, Leg ulcers, Skin Tumour, Acanthosis Nigricans
ENDO	Diabetes Insipidus, Diffuse Goitre, Gynaecomastia, Hirsutism, Hypoglycaemia, SIADH, Weight Loss
HAEM	Splenomegaly
PAEDS	Floppy infant
MISC	Anaphylaxis, Autoimmune associated diseases, Clubbing, Parotid Swelling, Splinter haemorrhages, Toxic agents and abnormal vitals, Toxicological causes of cardiac arrest
IMAGING	CHEST: Atelectasis, Hilar adenopathy, Hilar enlargement on CXR, Honeycomb lung, Increased interstitial markings, Mediastinal widening on mobile CXR, Pulmonary fibrosis, Pseudoinfiltrates on CXR, Pulmonary opacities on CXR, ABDO: Gas on abdominal X-ray, Kidney mass, BRAIN: Intracranial calcification, Intracranial structures with contrast, Ventriculomegaly, OTHER: Pseudofracture on X-Ray,
LABS	LOW: Anaemia, Hypocalcaemia, hypochloraemia, Hypomagnesaemia,  HIGH: Bilirubin and Jaundice, Hyperammonaemia, Hypercalcaemia, Hyperchloraemia, Hyperkalaemia, Hypermagnesaemia,  ACID BASE: Acid base disorders, Resp. acidosis, Resp. alkalosis, Creatinine, CRP, Dipstick Urinalysis, Laboratory Urinalysis, Liver function tests (LFTs), Pleural fluid analysis, Urea, Urea Creatinine Ratio, Uric acid, Urinalysis, Urine Electrolytes