125 Blood urea nitrogen (BUN)
Blood urea nitrogen (BUN) is the amount of nitrogen contained in the urea in blood. It is the same test as serum urea, expressed differently: BUN is common in the United States (mg/dL), and urea in Europe (mmol/L). BUN reflects the balance between urea production in the liver, from dietary protein and body protein breakdown, and its excretion by the kidneys. Together with creatinine it helps to distinguish between kidney disease, dehydration and other causes.
Units and conversion
Because one urea molecule (molar mass 60) contains two nitrogen atoms (28), urea (mg/dL) = BUN (mg/dL) × 2.14 and urea (mmol/L) = BUN (mg/dL) × 0.357. For example, BUN 14 mg/dL corresponds to urea 5.0 mmol/L.
High BUN
- Kidney causes: chronic kidney disease, acute kidney injury, urinary tract obstruction
- Pre-renal causes: dehydration, vomiting, diarrhoea, heart failure, low blood pressure
- Increased production: upper gastrointestinal bleeding, high-protein diet, corticosteroids, fever, burns, tissue breakdown
Low BUN
Low BUN occurs with low protein intake or malnutrition, severe liver disease, pregnancy and overhydration. On its own it is rarely significant.
if a high BUN comes with black stools or vomiting blood, very little urine, severe weakness, confusion or breathlessness.
Interpret BUN together with creatinine, eGFR, your fluid intake and medicines. Your doctor may repeat the test after normal hydration.
References
- MedlinePlus. BUN — blood test. U.S. National Library of Medicine. medlineplus.gov/ency/article/003474.htm
- Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2024 clinical practice guideline for the evaluation and management of chronic kidney disease. Kidney Int. 2024;105(4S):S117–S314. doi:10.1016/j.kint.2023.10.018
This article is for general information and does not replace a consultation with a doctor. Laboratory results must be interpreted by a physician together with symptoms and other tests.
Published on 9 May 2024