Type of test Blood

Normal findings

Adult: 10-20 mg/dL or 3.6-7.1 mmol/L (SI units)

Elderly: may be slightly higher than those of adult

 Child: 5-18 mg/dL

Infant: 5-18 mg/dL Newborn: 3-12 mg/dL

Cord: 21-40 mg/dL

Possible critical values

 > 100 mg/dL (indicates serious impairment of renal function)

Test explanation and related physiology

 The BUN measures the amount of urea nitrogen in the blood. Urea is formed in the liver as the end product of protein metabolism. During ingestion, protein is broken down into amino acids. In the liver, these amino acids are catabolized, and free ammonia is formed. The ammonia is combined to form urea, which is then deposited into the blood and transported to the kidneys for excretion. Therefore BUN is directly related to the metabolic function of the liver and the excretory function of the kidney. It serves as an index of the function of these organs. Patients who have elevated BUN levels are said to have azotemia.

Nearly all renal diseases cause inadequate excretion of urea, which causes the blood concentration to rise above normal. BUN also increases in conditions other than primary renal dis ease. For example, when excess amounts of protein are available for hepatic catabolism (from gastrointestinal [GI] bleeding), large quantities of urea are made.

BUN is interpreted in conjunction with the creatinine test. These tests are referred to as renal function studies.

The BUN/creatinine ratio is a good measurement of kidney and liver function. The normal adult range is 6 to 25, with 15.5 being the optimal adult value for this ratio.

Interfering factors

 • Changes in protein intake may affect BUN levels.

• Advanced pregnancy may cause increased BUN levels.

• Overhydration and underhydration will affect BUN levels.

• GI bleeding can cause increased BUN levels.

* Drugs that may cause increased BUN levels include allopurinol, aminoglycosides, cephalosporins, chloral hydrate, cisplatin, furosemide, guanethidine, indomethacin, methotrexate, methyldopa, nephrotoxic drugs (e.g., amphotericin B, aspirin, bacitracin, carbamazepine, colistin, gentamicin, methicillin, neomycin, penicillamine, polymyxin B, probenecid, vancomycin), propranolol, rifampin, spironolactone, tetracyclines, thiazide diuretics, and triamterene.

* Drugs that may cause decreased BUN levels include chloramphenicol and streptomycin.

Procedure and patient care

 • See inside front cover for Routine Blood Testing.

 • Fasting: no

• Blood tube commonly used: red

Abnormal findings

Increased levels

Prerenal causes

- Hypovolemia

- Shock

- Burns

- Dehydration

- Congestive heart failure

- Myocardial infarction

- GI bleeding

- Excessive protein ingestion

- Alimentary tube feeding

- Excessive protein catabolism

- Starvation

- Sepsis

 Renal causes

- Renal disease (e.g., glomerulonephritis, pyelonephritis, acute tubular necrosis)

- Renal failure

- Nephrotoxic drugs

 Postrenal azotemia

- Ureteral obstruction

- Bladder outlet obstruction

Decreased levels

- Liver failure

- Overhydration caused by fluid overload or syndrome of inappropriate antidiuretic hormone (SIADH)

- Negative nitrogen balance (e.g., malnutrition or malabsorption)

- Pregnancy

- Nephrotic syndrome

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