The Collaborative International Dictionary of English v.0.48:

BUN \BUN\ n. [acronym] (Med.)
   same as

   blood urea nitrogen; the concentration of nitrogen in blood
      present in the form of urea; -- used as a measure of
      kidney function.

   Note: Blood usually contains 10 to 15 mg of nitrogen per 100
         ml in the form of urea. --Stedman.
         [PJC]