How does the bun/creatinine ratio show whether or not you have dehydration?

BUN/Creatinine ratio. Blood urea nitrogen (BUN) is the breakdown of protein excreted via kidney. Creatinine is the muscle breakdown product also excreted by the kidney. BUN and creatinine can be measured by lab tests. Usually the BUN / creatinine ratio has to to be over 20 for a patient to be considered dehydrated or in "pre renal kidney failure.