Here is the explanation from icufaqs.com.
“Blood Urea Nitrogen” represents the amount of nitrogenous waste in the blood, which is supposed to be cleared by the kidneys. The BUN number always travels accompanied by its partner 1-1-7: creatinine (0.6 – 1.3 mg/dl), but it’s the creatinine number that is actually telling you directly how well the kidneys are working, since a rising BUN by itself can just indicate dehydration, just like a high sodium can. (A high admission hematocrit can be a clue too. Also the prunelike appearance. Then, again, some of us just look that way at baseline. Sigh.)
The thing to remember is that it's the creatinine that indicates if the kidneys are in trouble or not. High is bad. Someone told me once that if the creatinine increases by one whole number, it represents the loss of a third of the patient's kidney function, which means you can't do that very often!
So look at the BUN and creatinine as a ratio: normal would look like 12/ 1.0, right? A high BUN with a normal creatinine means a dry patient whose kidneys are still okay – if she gets hypotensively dry, her kidneys may become unhappy as a result of being under-perfused. Something like BUN of 70 with a creatinine of 1. High ratio. If the creatinine starts to rise, then real trouble is coming, because the kidneys are getting into trouble at the tissue level, maybe in the form of acute tubular necrosis, never a picnic. Might look like 70 / 3.0 – higher numbers, lower ratio. Comparison is everything, so take a look at a couple days' worth of chems and see if the creatinine has been going up, down or sideways.