What does the Blood Urea Nitrogen test measure?

Why is Blood Urea Nitrogen measured?

BUN is a first-line marker of renal excretory function and volume status. An isolated elevation may be prerenal (volume depletion, heart failure, gastrointestinal bleeding), intrinsic renal (acute or chronic kidney disease), or postrenal (urinary obstruction). When interpreted alongside creatinine — as the BUN:creatinine ratio — prerenal states (ratio typically >20:1) can be distinguished from intrinsic renal injury (ratio closer to 10–15:1). A low BUN suggests overhydration, malnutrition, severe liver disease, or pregnancy. Trends in serial BUN measurements guide fluid management, particularly in hospital and critical-care settings.

Reference ranges

Standard reference

Approximately 7–20 mg/dL (2.5–7.1 mmol/L); age- and hydration-dependent.

Functional medicine optimal

Practitioners often consider a BUN at the lower half of the reference interval (around 10–15 mg/dL) to indicate adequate hydration and normal renal clearance in ambulatory adults, though context — including dietary protein and muscle mass — shapes interpretation.

Units

mg/dL, mmol/L

Reference ranges vary by laboratory and assay platform. Interpret in context of the issuing laboratory's stated range.