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