BUN:Cr is a simple way to narrow down our differential diagnosis of renal diseases and other causes of these lab derangements. Note that creatinine is a very sensitive marker of renal dysfunction but is also increased in dehydration. Similarly, blood urea nitrogen may be lower than normal when the person is starved or does not take enough protein in diet. https://www.labpedia.net/blood-urea-nitrogen-creatinine-ratio-bun-creatinine-ratio/