An elevated ratio of blood urea nitrogen to serum creatinine was found in patients with severe urinary tract obstruction, and a lower ratio of urinary urea to creatinine was found in cases of renal ischemia. These findings are believed to be secondary, for the most part, to the slowing of the flow of urine through the renal tubule, with a resultant greater reabsorption of urea; this slowing, with obstruction, is from hydrostatic back pressure and, with renal ischemia, from a decrease in glomerular filtration rate. These ratios may be of practical value both in determining the relative role of obstruction in cases of impaired renal function and in differential function studies for renal hypertension.
Marshall S. Urea-Creatinine Ratio in Obstructive Uropathy and Renal Hypertension. JAMA. 1964;190(8):719-720. doi:10.1001/jama.1964.03070210025004