BUN to creatinine ratio - normal, high, low

The ideal ratio of BUN to creatinine falls between 10-to-1 and 20-to-1.

Both BUN and creatinine are used to test for chronic kidney disease. However, the BUN to creatinine ratio is not an indicator of chronic kidney disease. Assume a person has chronic kidney disease, his BUN goes up, his creatinine goes up too. When you calculate the ratio of BUN and creatinine, the ratio can be totally normal. It’s not an indicator for chronic kidney disease.

The ratio is however, linked to a few other conditions.

Acute kidney injury (previously termed acute renal failure)

The ratio is predictive of prerenal injury when BUN to Creatinine ratio exceeds 20 or when urea to Creatinine ratio exceeds 100. In prerenal injury, urea increases disproportionately to creatinine due to enhanced proximal tubular reabsorption that follows the enhanced transport of sodium and water.

Gastrointestinal bleeding

The ratio is useful for the diagnosis of bleeding from the gastrointestinal (GI) tract in patients who do not present with overt vomiting of blood.  In children, a BUN to creatinine ratio of 30 or greater has a sensitivity of 68.8% and a specificity of 98% for upper gastrointestinal bleeding.

A common assumption is that the ratio is elevated because of amino acid digestion, since blood (excluding water) consists largely of the protein hemoglobin and is broken down by digestive enzymes of the upper GI tract into amino acids, which are then reabsorbed in the GI tract and broken down into urea. However, elevated BUN:Cr ratios are not observed when other high protein loads (e.g., steak) are consumed. Renal hypoperfusion secondary to the blood lost from the GI bleed has been postulated to explain the elevated BUN:Cr ratio. However, other research has found that renal hypoperfusion cannot fully explain the elevation.

Advanced age

Because of decreased muscle mass, elderly patients may have an elevated BUN to Creatinine at baseline.

Other causes

Hypercatabolic states, high-dose glucocorticoids, and resorption of large hematomas have all been cited as causes of a disproportionate rise in BUN relative to the creatinine.

BUN to creatinine ratio is also expressed as BUN:Cr.

* The Content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.