BUN-to-creatinine ratio

Overview
In medicine, the BUN-to-creatinine ratio, also BUN-creatinine ratio, BUN/creatinine ratio or BUN:Cr, is the ratio of two serum laboratory values, the blood urea nitrogen (BUN) and serum creatinine. This terminology is used in the United States. In Canada and Europe, urea is used instead of BUN, so it is termed the urea-to-creatinine ratio, urea-creatinine ratio or urea/creatinine ratio. The interpretation and significance of the ratio is identical.

Acute renal failure
It has been found to be predictive of pre-renal failure, if the BUN-to-creatinine ratio is greater than 20 or the urea-to-creatinine ratio>0.10 and urea>10. In pre-renal failure, urea rises out of proportion to the creatinine due to enhanced proximal tubular reabsorption.

Gastrointestinal bleeds
It is useful for the diagnosis of upper gastrointestinal bleeding in patients who do not present with overt vomiting of blood.

In children, a BUN/creatinine ratio of 30 or above has a sensitivity of 68.8% for upper GI bleeding and a specificity of 98%.

Why the urea rises
The reason the urea concentration increases in upper GI bleeds is:
 * Blood, which consists largely of the protein hemoglobin, is broken down by digestive enzymes of the upper GI tract into amino acids.
 * The amino acids, which originate from the hemoglobin, are re-absorbed by the lower GI tract.
 * Urea is a break down product of amino acid catabolism; therefore, the "protein meal" from an upper GI bleed shows up in the blood as urea.

Elderly patients
Because of decreased muscle mass, elderly patients may have an elevated BUN-to-creatinine ratio at baseline.

External

 * Acute renal failure - American Academy of Family Physicians.