We may look at the BUN / Creatinine ratio to help determine the cause of renal failure. The ratio of BUN to creatinine is usually between 10:1 and 20:1. An increased ratio may be due to a condition that causes a decrease in the flow of blood to the kidneys, such as CHF or dehydration. It may also be seen with increased protein, from BI bleed, or increased protein in the diet. The ratio may be decreased with liver disease (due to a decrease in the formation of urea) and malnutrition.
*Note that BUN=Blood urea nitrogen which is essentially a measurement of urea. Urea is made in the liver as a by-product of protein metabolism.
| BUN: Cr | Location | Mechanism |
|---|---|---|
| >20:1 | Prerenal (before the kidney) | BUN reabsorption is increased. BUN is disproportionately elevated relative to creatinine in serum. Dehydration or hypoperfusion is suspected. |
| 10-20:1 | Normal or intrarenal (ATN) | Normal range. Can also be ATN. Some sources say the normal range is 10-15:1. Renal damage causes reduced reabsorption of BUN, therefore lowering the BUN:Cr ratio. In normal circumstances, Both BUN and Cr are filtered. As they pass through the renal tubule, BUN is reabsorbed from the PCT (proximal convoluted tubule) while Creatinine is not reabsorbed but instead, more is secreted into the tube in the DCT. In ATN, the reabsorption of BUN or secretion of Cr is decreased making the ratio to be normal. |
| <10:1 | Post-renal? |
*** A BUN/Cr ratio that is high when BUN and Creatinine are either low or at the lower range of normal is not going to be clinically significant.
The BUN/Cr ratio is normal at 10 to 15:1 in ATN (intrarenal), but is often greater than 20:1 in prerenal disease due to the increase in the passive reabsorption of urea that follows the enhanced proximal reabsorption of sodium and water . Thus, a high ratio is suggestive of prerenal disease as long as some other cause of a high ratio is not present.
As examples:
The BUN will rise out of proportion to the serum creatinine when urea production is increased due to GI bleed (upper somewhat more than lower), tissue breakdown, or glucocorticoid therapy.
The BUN/serum creatinine ratio can exceed 20:1 when loss of muscle mass in a chronically ill or older patient lowers creatinine production and, therefore, the serum creatinine concentration, independent of the GFR. However, this problem is chronic and cannot explain an acute rise in the BUN out of proportion to any change in the serum creatinine concentration.
In contrast to the potential utility of a high BUN/serum creatinine ratio, a normal ratio is of limited diagnostic utility. In particular, prerenal disease should not be excluded by a normal ratio since diminished urea production (due to decreased protein intake or underlying liver disease) can prevent the expected rise in BUN due to increased tubular reabsorption.