The ratio is predictive of prerenal injury when BUN:Cr exceeds 20
WHY IS IT ELEVATED? In prerenal injury, urea increases disproportionately to creatinine due to enhanced medullary urea re-absorption that follows the enhanced transport of sodium and water (due to hypo perfusion or dehydration). The recycled urea will draw water back into the interstitium by acting as an osmole. Creatinine is not readily re absorbed, hence why it does not concurrently increase.
Carvounis CP, Nisar S, Guro-Razuman S. Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure. Kidney Int. 2002;62:2223–9. [PubMed]