Expert: Newell R. Falkinburg, M.D., FACP - 8/30/2007

Question
QUESTION: I am a white 58 year old female taking arimidex, zocor, and avapro for high blood pressure.  I had routine blood work done and had a high bun/cratine ratio of 23.  The lab says normal the normal range is 6 - 22.  Everything else was within normal range. My creatinine was 0.6 and my urea nitrogen was 14.  What could cause the ratio to be high and what follow up is necessary?  Thank you very much.

ANSWER: Dear Nancy,

First of all, you should know that with a BUN of 14 and a Creatinine of o.6 mg% your kidney function is completely normal.

Usually, the BUN/Cr ratio is 10-15 to one.  Because urea is often reabsorbed by the kidney(creatinine is not) it's concentration occasionally will rise disproportionately to creatinine. Also, urea is increased by the dietary intake of protein so one's diet can effect it's level in the blood.

The following are some of the clinical issues that can give one an increased BUN/Cr ratio:

1. Urinary tract obstruction
     It would be highly unlikely that you have this   because both the BUN & creatinine would be elevated, the BUN disproportionately so.
2.  Dehydration
      This clinical condition slows the flow of fluid through the kidney thereby facilitating the reabsorption of urea. Creatinine is not reabsorbed.  This scenario is unlikely also.
3.  High protein diet
       Urea is the result of protein metabolism and lots of protein in the diet will produce lots of urea in the blood. This is possible.
4.  Small muscle mass
       Creatinine comes from muscle metabolism and if you are chronically ill and not consuming adequate calories you will waste muscle and your creatinine production will fall. A low  blood creatinine results and the BUN/Cr ratio rises. This is also possible but I'm not is a position to make the assessment.
5. Blood in the intestinal tract  (analogous to a high protein diet) There will be a rise in the BUN causing the ratio to rise. You might want to consider having your stools checked for blood to be on the save side.

6. Catabolic states
     Diseases associated with a lot of tissue breakdown such as cancer, chemotherapy, infections etc result is the body's consuming some of its own tissues for energy.  This, also, results in he BUN rising.  I assume you have or had breast cancer and are on arimidex for that but I cannot, over the Internet, assess your case in that detail.  I would say that it's possible for this to contribute to a rise in the BUN/Cr ratio.

Clinically, the most common cause of a high BUN/Cr ratio is a high protein diet.

This ratio, however, taken in the setting of a normal BUN and creatinine is almost never of any clinical consequence.  Even blood in the G.I. tract will nearly always cause the BUN to rise above normal.  Your's is fine. Additionally, your ratio is only one point above the lab's reference upper limit. This suggests that any influence is minor.

Hope this complicated jargon helps you.  It's difficult to understand as well to explain in an encapsulated and lucid form.

Thanks for the question. Please feel free to follow up if it would help you.

Sincerely,

Dr. Falkinburg


     



---------- FOLLOW-UP ----------

QUESTION: Thank you so much for your thoughtful thorough answer.  Just to clarify about the arimidex, I was treated for early stage breast cancer 3years ago, with radiation and chemotherapy.  I am taking arimidex for five years, and as far as I know I no longer have breast cancer.  Do you think this ratio in light of all other blood work being completely normal could be an indeication that the cancer has spread?  I am definately consuming adequete calories, so small muscle mass doesn't apply to me.  Thank you again.

Answer
Dear Nancy,

No, I do not feel that this is at all related to any recurrence of your breast cancer.  I think that the odds are highly in favor of this being do to the protein in your diet which raises the BUN relative to the creatinine. I am comfortable with this because, even though the ratio is up (and barely so), both the BUN and creatinine are well within the normal range. You might consider having your stools checked for blood but I would be surprised if that were positive.  Best, however, to be safe.

Thanks for the follow up.

Sincerely,

Dr. Falkinburg