Biomarker: BUN, Blood

Overview

Blood urea nitrogen (BUN) measures the amount of urea nitrogen in blood. Urea is produced primarily by the liver when the body metabolizes protein and is removed from the blood by the kidneys. High BUN may occur with reduced kidney filtration, dehydration, gastrointestinal bleeding, high protein intake, or certain medications, while low BUN may occur with severe liver disease, malnutrition, low protein intake, or overhydration. BUN is clinically useful for assessing kidney function and hydration status, but it should be interpreted with creatinine, estimated glomerular filtration rate, symptoms, and other clinical findings.

Clinical Use Cases

Assessing kidney function and identifying possible azotemia or impaired renal filtration.
Monitoring known acute or chronic kidney disease.
Evaluating hydration status and possible volume depletion.
Interpreting suspected gastrointestinal bleeding, particularly when considered with other clinical findings.
Evaluating patients with conditions that may affect renal perfusion, including heart failure or shock.
Including in basic or comprehensive metabolic panels for general medical assessment.
Supporting assessment of nutritional or hepatic disorders when BUN is unexpectedly low.

Specimen Types

Serum from venous blood.
Plasma from venous blood.
Whole blood may be used with some validated point-of-care or specialized methods, although serum or plasma is typical for routine laboratory testing.

Measurement Methods

Enzymatic urease methods, in which urease converts urea to ammonia and carbon dioxide.
Urease-glutamate dehydrogenase coupled assays, which measure the change in NADH or NADPH absorbance, commonly at 340 nm.
Enzymatic colorimetric methods that measure a colored reaction product formed after urease activity.
Historical diacetyl, or Fearon, photometric methods, which are less commonly used than enzymatic methods.

Test Preparation and Influencing Factors

BUN testing alone generally requires no special preparation. Fasting may be required if other tests collected at the same time require it.
Follow the ordering clinician’s instructions regarding medications. Patients should not stop or change medications without medical advice.
Dehydration or reduced circulating blood volume can increase BUN.
A high-protein diet or recent high protein intake can increase BUN.
Gastrointestinal bleeding and increased tissue breakdown can raise BUN.
Severe liver disease, malnutrition, starvation, low protein intake, and overhydration can lower BUN.
Age, sex, pregnancy, illness severity, and laboratory-specific reference intervals can affect interpretation.
BUN is not specific for kidney disease and should be interpreted with creatinine, estimated glomerular filtration rate, hydration status, and the clinical context.
Serum or plasma specimens should be collected and handled according to the laboratory’s validated requirements; sodium fluoride tubes may interfere with some urease-based methods.