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Introduction ⬇

Urea forms in the liver and, along with CO2, constitutes the final product of protein metabolism. The amount of excreted urea varies directly with dietary protein intake, increased excretion in fever, diabetes, and increased adrenal gland activity.

The test for blood urea nitrogen (BUN), which measures the nitrogen portion of urea, is used as an index of glomerular function in the production and excretion of urea. Rapid protein catabolism and impairment of kidney function will result in an elevated BUN level. The rate at which the BUN level rises is influenced by the degree of tissue necrosis, protein catabolism, and the rate at which the kidneys excrete the urea nitrogen. A markedly increased BUN is conclusive evidence of severe impaired glomerular function. In chronic renal disease, the BUN level correlates better with symptoms of uremia than does the serum creatinine.

Normal Findings ⬆ ⬇

Adults: 6–20 mg/dL or 2.1–7.1 mmol/L

Older adults (older than 60 years): 8–23 mg/dL or 2.9–8.2 mmol/L

Children: 5–18 mg/dL or 1.8–6.4 mmol/L

Clinical Alert

Critical value for BUN is >100 mg/dL (>35 mmol/L)

Procedure ⬆ ⬇

  1. Obtain a 5-mL venous blood sample (red-topped tube). Serum is preferred.

  2. Observe standard precautions.

Clinical Implications ⬆ ⬇

  1. Increased BUN levels (azotemia) occur in the following conditions:

    1. Impaired kidney function caused by the following conditions:

      1. Heart failure

      2. Salt and water depletion

      3. Shock

      4. Stress

      5. Acute MI

    2. CKD such as glomerulonephritis and pyelonephritis

    3. Urinary tract obstruction

    4. Hemorrhage into GI tract

    5. Diabetes with ketoacidosis

    6. Excessive protein intake or protein catabolism as occurs in burns or cancer

    7. Anabolic steroid use

  2. Decreased BUN levels are associated with the following conditions:

    1. Liver failure (severe liver disease), such as that resulting from hepatitis, drugs, or poisoning

    2. Acromegaly

    3. Malnutrition, low-protein diets

    4. Impaired absorption (celiac disease)

    5. Nephrotic syndrome (occasional)

    6. Syndrome of inappropriate antidiuretic hormone (SIADH)

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain test purpose and blood-drawing procedure. Assess dietary history.

  2. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

  1. Review test outcome and monitor as appropriate for impaired kidney function. Modify the nursing care plan as needed.

  2. In patients with an elevated BUN level, fluid and electrolyte regulation may be impaired.

  3. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Clinical Alert

If a patient is confused, disoriented, or has convulsions, the BUN level should be checked. If the level is high, it may help to explain these signs and symptoms

Interfering Factors ⬆

  1. A combination of a low-protein and high-carbohydrate diet can cause a decreased BUN level.

  2. The BUN is normally lower in children and women because they have less muscle mass than adult men.

  3. Decreased BUN values normally occur in late pregnancy because of increased plasma volume (physiologic hydremia).

  4. Older persons may have an increased BUN when their kidneys are not able to concentrate urine adequately.

  5. IV feedings only may result in overhydration and decreased BUN levels.

  6. Many drugs may cause increased or decreased BUN levels.