A ratio >20 suggests pre-renal azotemia — the kidneys are underperfused but intrinsically intact. Common causes include dehydration, heart failure, shock, and upper GI bleeding (where digested blood protein increases BUN). High protein intake, corticosteroids, and tetracyclines can also elevate the ratio.

A ratio <10 may indicate liver disease (decreased urea production), malnutrition, low protein diet, SIADH (dilution), pregnancy, or rhabdomyolysis (elevated creatinine from muscle breakdown with relatively normal BUN).

The BUN/Cr ratio is a screening tool, not a definitive test. BUN is affected by diet, liver function, GI bleeding, catabolic state, and medications. The ratio should be interpreted alongside clinical context, urinalysis, FENa, and other tests. It is most useful in the initial evaluation of elevated creatinine.