BUN — Normal Range & Interpretation

Clinical Context

Blood urea nitrogen measures the nitrogen portion of urea, the waste product formed when the liver breaks down protein. The kidneys filter urea and excrete it in urine, so BUN reflects both renal function and protein metabolism. Nurse practitioners pair BUN with creatinine to distinguish prerenal from intrinsic renal causes of kidney dysfunction and to assess hydration status in acute care and primary care settings.

Elevations point to dehydration, acute kidney injury, chronic kidney disease, gastrointestinal bleeding, high-protein diets, catabolic states, and medications such as aminoglycosides, NSAIDs, and ACE inhibitors. Depressed values signal malnutrition, low protein intake, advanced liver disease, pregnancy, and overhydration. A BUN-to-creatinine ratio above 20:1 indicates prerenal azotemia from volume depletion or GI bleed, while a ratio near 10:1 to 15:1 suggests intrinsic renal disease.

Board-style questions on BUN pair interpretation within clinical vignettes rather than recall of the isolated number. Expect questions linking an elevated BUN with dehydration in an older adult on diuretics, prerenal azotemia after GI bleeding, or worsening renal function in a patient started on lisinopril. The exam also tests recognition of the BUN-to-creatinine ratio and the impact of protein intake and hepatic function on baseline values.

Related Labs

• Sodium (Na) — mEq/L
• Potassium (K) — mEq/L
• Chloride (Cl) — mEq/L
• CO2 (Bicarbonate) — mEq/L
• Creatinine — mg/dL

Sources

• MedlinePlus