Blood test is a very common examination in diagnosis of kidney disease. The tests for BUN (blood urea nitrogen) and creatinine are used to monitor kidney functions. These two substances are normal metabolic waste products that are excreted by the kidneys. Urea is a byproduct of protein breakdown while creatinine is the waste product of normal muscle activity. Normally, these two wastes are removed by kidneys. Once kidneys cannot function well, they will build up in the body, causing high BUN and creatinine levels. Both of them can be easily detected on blood tests.