Understanding BUN:Cr Ratios for the CCRN Exam! -A ratio greater than 20:1 suggests pre-renal causes like volume depletion, CHF, or decreased perfusion. -A normal ratio (10–15:1) points toward intra-renal injury such as ATN or glomerulonephritis. -A variable ratio raises concern for post-renal obstruction like BPH, stones, or tumors. The ratio alone doesn’t diagnose the problem—but it’s a fast, valuable clue when paired with clinical context, labs, and a good history. . Check out the ICU STARTER