Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to fill with blood.

A needle is inserted into the vein, and the blood is collected in an airtight vial or a syringe. During the procedure, the tourniquet is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants and young children, the area is cleaned with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. A bandage may be applied to the puncture site if there is any continued bleeding.

Some drugs affect BUN levels. Before having this test, make sure the health care provider knows which medications you are taking.

Drugs that can increase BUN measurements include:

• Allopurinol
• Aminoglycosides
• Amphotericin B
• Bacitracin
• Carbamazepine
• Cephalosporins
• Chloral hydrate
• Cisplatin
• Colistin
• Furosemide
• Gentamicin
• Guanethidine
• High-dose aspirin
• Indomethacin
• Methicillin
• Methotrexate
• Methyldopa
• Neomycin
• Penicillamine
• Polymyxin B
• Probenecid
• Propranolol
• Rifampin
• Spironolactone
• Tetracyclines
• Thiazide diuretics
• Triamterene
• Vancomycin

Drugs that can decrease BUN measurements include:

• Chloramphenicol
• Streptomycin

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

The BUN test is often done to check kidney function.

7 - 20 mg/dl. Note that normal values may vary among different laboratories.

Higher-than-normal levels may be due to:

• Congestive heart failure
• Excessive protein levels
• Gastrointestinal bleeding
• Hypovolemia
• Heart attack
• Kidney disease, including glomerulonephritis , pyelonephritis , and acute tubular necrosis
• Kidney failure
• Shock
• Urinary tract obstruction

Lower-than-normal levels may be due to:

• Liver failure
• Low protein diet
• Malnutrition
• Over-hydration

Additional conditions under which the test may be done include:

• Acute nephritic syndrome
• Alport syndrome
• Atheroembolic kidney disease
• Dementia due to metabolic causes
• Diabetic nephropathy/sclerosis
• Digitalis toxicity
• Epilepsy
• Generalized tonic-clonic seizure
• Goodpasture syndrome
• Hemolytic-uremic syndrome (HUS)
• Hepatokidney syndrome
• Interstitial nephritis
• Lupus nephritis
• Malignant hypertension (arteriolar nephrosclerosis)
• Medullary cystic kidney disease
• Membranoproliferative GN I
• Membranoproliferative GN II
• Type 2 diabetes
• Prerenal azotemia
• Primary amyloidosis
• Secondary systemic amyloidosis
• Wilms' tumor

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks are slight but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

For people with liver disease , the BUN level may be low even if the kidneys are normal.