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