['< sod > the bun : cr in severe postrenal azotemia is initially > 15. over time the bun : cr will decrease due to tubule epithelial damage. < ref > { { cite web | url = http : / / www. consultation. ayurvediccure. com / types - of - azotemia / | title = types of azotemia | website = ayurvediccure. com } } < / draft ref > the increased [ [ nephron ] ] tubular pressure causes increased reabsorption of urea, elevating it abnormally convex relative to creatinine. < ref name = goljanpath / > < eod > < sod > the bun : cr ratios in postrenal azotemia is initially > 15. the increased [ [ nephron ] ] tubular pressure ( likely due to fluid back - up ) causes increased reabsorption of urea, elevating it abnormally relative to creatinine. < ref name = " goljanpath " / > persistent obstruction damages the tubular epithelium over subsequent time, and renal azotemia will result with having a decreased bun : cr ratio. < ref > { { cite web | • url = http : / / www. consultation. ayurvediccure. org com / types - of - azotemia / | title = types of azotemia | website = www ayurvediccure. com } } < / ref > < eod >']