Urea-Creatinine Ratio
Basics
Urea and creatinine are both found in the bloodstream. They reach the glomerulus and they are both filtered. They both pass from the “Glomerulus capillary” into “Bowman’s capsule.” However, urea is reabsorbed from the tubules back into the bloodstream. Creatinine is not reabsorbed and is then excreted into the urine.
Normal Urea: 2.1 – 7.1 mmol/L (from NHLS)
Normal Creatinine: 64 – 104 umol/L (from NHLS)
NB: To convert from umol/L into mmol/L, you need to divide by 1000. Hence, the normal creatinine is 0.062 mmol/L – 0.104mmol/L. This is important so that both the urea and creatinine are assessed using the same unit.
In the United States, doctors use Blood Urea Nitrogen (BUN) which is usually stated as mg/dL. In other countries, such as South Africa, we use mmol/L or umol/L. This means that when you read books/articles, you may find the following:
- BUN:Cr (mg/dL : mg/dL) – United States
- Urea:Cr (mmol/L : umol/L) – South Africa
Hence, there are also DIFFERENT ratios to use for each unit. Use the one with the correct units for your clinical setting.
| Location | Urea:Cr (South Africa) | BUN:Cr | Causes |
|---|---|---|---|
| Pre-Renal – The urea is reabsorbed and hence increased in greater amounts than Creatinine. The ratio becomes INCREASED. | >100:1 | >20:1 | -“Pre-renal uremia” -Dehydration - Vomiting - Diarrhea - After GI hemorrhage - Protein rich diet |
| Normal or Post-Renal | 40-100:1 | 10-20:1 | |
| Renal (AKI or Acute Tubular Necrosis) – The urea is no longer absorbed as much as in pre-renal, and hence the ratio between Urea and Creatinine gets DECREASED. | <40:1 | <10:1 | -Intrinsic renal damage(Acute tubular Necrosis) - Low protein intake - Starvation/Malnutrition - Severe liver disease |
Other investigations to order:
- Urine Na (<20 Pre-renal and >40 in Acute Tubular Necrosis)
- Urine osmolality (>500 in pre-renal and <350 in acute tubular necrosis)
- Urine-specific gradient (>1.020 in pre-renal and < 1.010 in Acute tubular necrosis)
- Fractional excretion of sodium (<1% in pre-renal and >2% in acute tubular necrosis)
- Urinalysis: You will also need to perform a urinalysis (will appear normal/hyaline casts for pre-renal and will have granular casts/tubular epithelial cells in acute tubular necrosis.
Examples
Normal
On the example above, the Urea is 6.1 mmol/L and Creatinine is 79 umol/L (0.079mmol/L). Therefore, to find the Urea: Creatinine ratio we do the following calculation: 6.1 divided by 0.079 (same mmol/L units), which results in 77.22. This value lies between 40 – 100 and hence it is a normal value. This is also supported clinically, this patient did not have any condition affecting their urea or creatinine.
Pre-Renal
However, let us assume that the patient above becomes progressively more dehydrated due to vomiting. Their urea rises to 11 mmol/L and their creatinine to 83 umol/L (0.083 mmol/L). This means that the new Urea: Creatinine ratio is 11 divided by 0.083, which is 132.53. This value is >100, and by using the table above we can see that this coincides with pre-renal uremia.
Renal
Continuing with the example above, the doctors fail to rehydrate the patient. The patient becomes severely dehydrated and begins to have a renal injury due to renal hypoperfusion. The urea rises to 15mmol/L and the creatinine rises to 489umol/L (0.489mmol/L). This means that the new Urea: Creatinine ratio is 15 divided by 0.489 which is 30.68. This value is < 40, and by using the table above we can see that this coincides with an acute kidney injury or acute tubular necrosis. The other investigations will assist you even further.
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