Urea Nitrogen, Blood

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Urea Nitrogen, Blood

Synonym/acronym: BUN.

Common use

To assist in assessing for renal function toward diagnosing disorders such as kidney failure and dehydration. Also used in monitoring the effectiveness of therapeutic interventions such as hemodialysis.


Serum (1 mL) collected in a gold-, red-, or red/gray-top tube. Plasma (1 mL) collected in a green-top (heparin) tube is also acceptable.

Normal findings

(Method: Spectrophotometry)
AgeConventional UnitsSI Units (Conventional Units × 0.357)
Newborn–3 yr5–17 mg/dL1.8–6.1 mmol/L
4–13 yr7–17 mg/dL2.5–6.1 mmol/L
14 yr–adult8–21 mg/dL2.9–7.5 mmol/L
Adult older than 90 yr10–31 mg/dL3.6–11.1 mmol/L


Unlike fats and carbohydrates, protein cannot be stored by the body. The amino acids and nitrogen used to make proteins are either obtained from dietary sources or from the normal turnover of aging cells in the body. Urea is a nonprotein nitrogen (NPN) compound formed in the liver from ammonia and excreted by the kidneys as an end product of protein metabolism. Other NPN compounds excreted by the kidneys include uric acid and creatinine. Blood urea nitrogen (BUN) levels reflect the balance between the amount of nitrogen ingested and excreted which is a representation of overall protein metabolism. BUN and creatinine values are commonly evaluated together. The normal BUN/creatinine ratio is 15:1 to 24:1. (e.g., if a patient has a BUN of 15 mg/dL, the creatinine should be approximately 0.6 to 1 mg/dL). BUN is used in the following calculation to estimate serum osmolality: (2 × Na+) + (glucose/18) + (BUN/2.8).

This procedure is contraindicated for



  • Assess nutritional support
  • Evaluate hemodialysis therapy
  • Evaluate hydration
  • Evaluate liver function
  • Evaluate patients with lymphoma after chemotherapy (tumor lysis)
  • Evaluate renal function
  • Monitor the effects of drugs known to be nephrotoxic or hepatotoxic

Potential diagnosis

Increased in

  • Acute renal failure (related to decreased renal excretion)
  • Chronic glomerulonephritis (related to decreased renal excretion)
  • Congestive heart failure (related to decreased blood flow to the kidneys, decreased renal excretion, and accumulation in circulating blood)
  • Decreased renal perfusion (reflects decreased renal excretion and increased blood levels)
  • Diabetes (related to decreased renal excretion)
  • Excessive protein ingestion (related to increased protein metabolism)
  • Gastrointestinal (GI) bleeding (excessive blood protein in the GI tract and increased protein metabolism)
  • Hyperalimentation (related to increased protein metabolism)
  • Hypovolemia (related to decreased blood flow to the kidneys, decreased renal excretion, and accumulation in circulating blood)
  • Ketoacidosis (dehydration from ketoacidosis correlates with decreased renal excretion of urea nitrogen)
  • Muscle wasting from starvation (related to increased protein metabolism)
  • Neoplasms (related to increased protein metabolism or to decreased renal excretion)
  • Nephrotoxic agents (related to decreased renal excretion and accumulation in circulating blood)
  • Pyelonephritis (related to decreased renal excretion)
  • Shock (related to decreased blood flow to the kidneys, decreased renal excretion, and accumulation in circulating blood)
  • Urinary tract obstruction (related to decreased renal excretion and accumulation in circulating blood)

Decreased in

    Inadequate dietary protein (urea nitrogen is a by-product of protein metabolism; less available protein is reflected in decreased BUN levels) Low-protein/high-carbohydrate diet (urea nitrogen is a by-product of protein metabolism; less available protein is reflected in decreased BUN levels) Malabsorption syndromes (urea nitrogen is a by-product of protein metabolism; less available protein is reflected in decreased BUN levels) Pregnancy Severe liver disease (BUN is synthesized in the liver, so liver damage results in decreased levels)

Critical findings

  • Adults
  • Greater than 100 mg/dL (SI: Greater than 35.7 mmol/L) (nondialysis patients)
  • Children
  • Greater than 55 mg/dL (SI: Greater than 19.6 mmol/L) (nondialysis patients)
  • Note and immediately report to the health-care provider (HCP) any critically increased or decreased values and related symptoms especially fluid imbalance. It is essential that a critical finding be communicated immediately to the requesting health-care provider (HCP). A listing of these findings varies among facilities.

  • Timely notification of a critical finding for lab or diagnostic studies is a role expectation of the professional nurse. Notification processes will vary among facilities. Upon receipt of the critical value the information should be read back to the caller to verify accuracy. Most policies require immediate notification of the primary HCP, Hospitalist, or on-call HCP. Reported information includes the patient’s name, unique identifiers, critical value, name of the person giving the report, and name of the person receiving the report. Documentation of notification should be made in the medical record with the name of the HCP notified, time and date of notification, and any orders received. Any delay in a timely report of a critical finding may require completion of a notification form with review by Risk Management.

  • A patient with a grossly elevated BUN may have signs and symptoms including acidemia, agitation, confusion, fatigue, nausea, vomiting, and coma. Possible interventions include treatment of the cause, administration of IV bicarbonate, a low-protein diet, hemodialysis, and caution with respect to prescribing and continuing nephrotoxic medications.

Interfering factors

  • Drugs, substances, and vitamins that may increase BUN levels include acetaminophen, alanine, aldatense, alkaline antacids, amphotericin B, antimony compounds, arsenicals, bacitracin, bismuth subsalicylate, capreomycin, carbenoxolone, carbutamide, cephalosporins, chloral hydrate, chloramphenicol, chlorthalidone, colistimethate, colistin, cotrimoxazole, dexamethasone, dextran, diclofenac, doxycycline, ethylene glycol, gentamicin, guanethidine, guanoxan, ibuprofen, ifosfamide, ipodate, kanamycin, mephenesin, metolazone, mitomycin, neomycin, phosphorus, plicamycin, tertatolol, tetracycline, triamterene, triethylenemelamine, viomycin, and vitamin D.
  • Drugs that may decrease BUN levels include acetohydroxamic acid, chloramphenicol, fluorides, paramethasone, phenothiazine, and streptomycin.

Nursing Implications and Procedure

Potential nursing problems

ProblemSigns & SymptomsInterventions
Infection (Related to uremia and decreased immune response; venous catheters; Foley catheters; endotracheal tubes)Temperature; elevated white cell count; cloudy urine; sediment in urine; blood in urineMonitor urinary output; assess urine color, odor, presence of blood; monitor and trend temperature and white cell count; obtain urine for culture and sensitivity as required; administer prescribed antibiotics; avoid long-term indwelling catheters; encourage frequent personal and oral hygiene; encourage use of gentle soaps; assist in preventing skin breakdown; use meticulous care and sterile technique in provision of care for peripheral or venous catheters
Renal function (Related to renal ischemia associated with shock, sepsis, hypovolemia; postoperative injury; trauma; nephrotoxic drugs [aminoglycoside, heavy metals, radiographic contrast]; renal vascular occlusion; hemolytic transfusion reaction; decreased cardiac output; tubular necrosis; obstruction; tumor; medications [NSAID, ACE, immunosuppressants, antineoplastics, antifungals])Increased BUN; increased creatinine; decreased creatinine clearance; increased urine specific gravity (greater than 1.029); hematuria; proteinuria; decreased urine output less than 400 mL/day (with adequate intake and no fluid loss); weight gain; elevated potassium; elevated phosphate; decreased calcium; decreased sodium; increased magnesium; metabolic acidosis; decreased HGB/HCTMonitor, record, and trend intake and output; monitor urine specific gravity; monitor and trend renal specific urine and blood studies, BUN, creatinine, sodium, potassium, magnesium, pH, urinalysis, HGB/HCT; monitor and trend weight daily; assess and monitor for edema, JVD, hypertension, adventitious breath sounds, impaired gas exchange; use pulse oximetry; administer prescribed oxygen; administer prescribed hemodialysis; administer prescribed medications (diuretics); administer prescribed fluids; consider renal function with antibiotic administration
Cardiac output (Related to excess fluid volume; pericarditis; electrolyte imbalance; toxin accumulation; dysrythmias; altered cardiac muscle contractility secondary to heart failure)Weak peripheral pulses; slow capillary refill; decreased urinary output; cool, clammy skin; tachypnea; dyspnea; altered level of consciousness; abnormal heart sounds; fatigue; hypoxia; loud holosystolic murmur; EKG changes; increased JVDAssess peripheral pulses and capillary refill; monitor blood pressure and check for orthostatic changes; assess respiratory rate, breath sounds, and orthopnea; assess skin color and temperature; assess level of consciousness; monitor urinary output; use pulse oximetry to monitor oxygenation; monitor EKG; assess for third heart sound (indicative of HF or pericarditis); administer ordered inotropic and peripheral vasodilator medications, nitrates; provide oxygen administration; administer as prescribed (sodium bicarbonate, glucose, insulin drip, potassium excretion resin, calcium salt)
Fluid volume (Related to excess fluid and sodium intake; compromised renal function)Excess: edema, shortness of breath, increased weight, ascites, rales, rhonchi, and diluted laboratory values; distended neck veins; tachycardia; restlessness; presence of S3 heart soundRecord daily weight and monitor trends; record accurate intake and output; monitor laboratory values that reflect alterations in fluid status (potassium, blood urea nitrogen, creatinine, calcium, hemoglobin, hematocrit, sodium); manage underlying cause of fluid alteration; monitor urine characteristics and respiratory status; establish baseline assessment data; assess and trend heart rate and blood pressure; assess for symptoms of fluid overload such as JVD, shortness of breath, dyspnea, crackles; provide low sodium diet; administer prescribed diuretic; administer prescribed antihypertensive; elevate feet when sitting; monitor oxygenation with pulse oximetry; administer oxygen as appropriate; elevate the head of the bed; administer prescribed antihypertensives; administer IV medications with the least amount of fluid; prepare for hemodialysis as appropriate


  • Positively identify the patient using at least two unique identifiers before providing care, treatment, or services.
  • Patient Teaching: Inform the patient this test can assist in assessing kidney function.
  • Obtain a history of the patient’s complaints, including a list of known allergens, especially allergies or sensitivities to latex.
  • Obtain a history of the patient’s genitourinary and hepatobiliary systems, symptoms, and results of previously performed laboratory tests and diagnostic and surgical procedures.
  • Obtain a list of the patient’s current medications, including herbs, nutritional supplements, and nutraceuticals (see Effects of Natural Products on Laboratory Values online at DavisPlus).
  • Review the procedure with the patient. Inform the patient that specimen collection takes approximately 5 to 10 min. Address concerns about pain and explain that there may be some discomfort during the venipuncture.
  • Sensitivity to social and cultural issues, as well as concern for modesty, is important in providing psychological support before, during, and after the procedure.
  • Note that there are no food, fluid, or medication restrictions unless by medical direction.


  • Potential complications: N/A
  • Avoid the use of equipment containing latex if the patient has a history of allergic reaction to latex.
  • Instruct the patient to cooperate fully and to follow directions. Direct the patient to breathe normally and to avoid unnecessary movement.
  • Observe standard precautions, and follow the general guidelines in Patient Preparation and Specimen Collection. Positively identify the patient, and label the appropriate specimen container with the corresponding patient demographics, initials of the person collecting the specimen, date, and time of collection. Perform a venipuncture.
  • Remove the needle and apply direct pressure with dry gauze to stop bleeding. Observe/assess venipuncture site for bleeding or hematoma formation and secure gauze with adhesive bandage.
  • Promptly transport the specimen to the laboratory for processing and analysis.


  • Inform the patient that a report of the results will be made available to the requesting HCP, who will discuss the results with the patient.
  • Monitor intake and output for fluid imbalance in renal dysfunction and dehydration.
  • Nutritional Considerations: Greater than 100 nitrogen balance is commonly used as a nutritional assessment tool to indicate protein change. In healthy individuals, protein anabolism and catabolism are in equilibrium. During various disease states, nutritional intake decreases, resulting in a negative balance. During recovery from illness and with proper nutritional support, the nitrogen balance becomes positive. BUN is an important analyte to measure during administration of total parenteral nutrition (TPN). Educate the patient, as appropriate, in dietary adjustments required to maintain proper nitrogen balance. Inform the patient that the requesting HCP may prescribe TPN as part of the treatment plan.
  • Nutritional Considerations: An elevated BUN can be caused by a high-protein diet or dehydration. Unless medically restricted, a healthy diet consisting of the five food groups of the USDA’s MyPlate dietary guide should be consumed daily. Water consumption should include six to eight 8-oz glasses of water per day, or water consumption equivalent to half of the body’s weight in fluid ounces (32 fl oz = 1 qt; 34 fl oz = 1 L).
  • Recognize anxiety related to test results.
  • Depending on the results of this procedure, additional testing may be performed to evaluate or monitor progression of the disease process and determine the need for a change in therapy. Evaluate test results in relation to the patient’s symptoms and other tests performed.
  • Patient Education

    • Discuss the implications of abnormal test results on the patient’s lifestyle.
    • Provide teaching and information regarding the clinical implications of the test results, as appropriate.
    • Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP.
    • Answer any questions or address any concerns voiced by the patient or family.
    • Teach the patient how to accurately measure and record urine output.
  • Expected Patient Outcomes

    • Knowledge
    • States understanding that hemodialysis may be necessary if renal function fails to improve
    • States the importance of adhering to the specified fluid limit
    • Skills
    • Demonstrates proficiency in tracking and documenting intake and output
    • Demonstrates proficiency in taking and documenting temperature
    • Attitude
    • Complies with the request for frequent personal and oral hygiene to decrease infection risk
    • Complies with the request to take prescribed medication as ordered

Related Monographs

  • Related tests include anion gap, antimicrobial drugs, biopsy kidney, calcium, calculus kidney stone panel, CT spleen, creatinine, creatinine clearance, cytology urine, cystoscopy, electrolytes, gallium scan, glucose, glycated hemoglobin, 5–HIAA, IVP, ketones, magnesium, MRI venography, microalbumin, osmolality, oxalate, phosphorus, protein total and fractions, renogram, US abdomen, US kidney, UA, urea nitrogen urine, and uric acid.
  • Refer to the Genitourinary and Hepatobiliary systems tables at the end of the book for related tests by body system.
Handbook of Laboratory and Diagnostic Tests, © 2013 Farlex and Partners