Urea is a nonprotein nitrogenous compound that is formed in the liver from ammonia. Although urea diffuses freely into both extracellular and intracellular fluid, it is ultimately excreted by the kidneys. Blood urea levels reflect the balance between production and excretion of urea. Changes in protein intake, fluid balance, liver function, and renal excretion affect blood urea levels. Specific causes of alterations are listed in Table 5-7.

Blood urea analysis involves measurement of nitrogen; the result is expressed as urea nitrogen. Nitrogen contributes 46.7 percent of the total weight of urea. The concentration of urea can be calculated by multiplying the urea nitrogen result by 2.14.¹⁹

• Therapy with drugs known to alter urea nitrogen levels (see Table 5-7)

• Known or suspected disorder associated with impaired renal function, as indicated by increased levels:
  • Obstructive, inflammatory, or toxic damage to the kidneys, nephron loss caused by aging, or extrarenal conditions that reduce the glomerular filtration rate (GFR) increase retention of urea.
• Monitoring for the effects of disorders associated with altered fluid balance:
  • Dehydration or hypovolemia caused by vomiting, diarrhea, hemorrhage, or inadequate fluid intake raises the urea nitrogen.
  • Fluid overload decreases the urea nitrogen if renal function is adequate.
• Known or suspected liver disease as indicated by decreased levels caused by the liver's inability to convert ammonia to urea (80 percent of liver function may be lost before this is evident)
• Monitoring for effects of drugs known to be nephrotoxic or hepatotoxic
• Monitoring of response to various disorders known to result in altered urea nitrogen levels (see Table 5-7)

Nursing Care Before the Procedure

Client preparation is the same as that for any test involving collection of a peripheral blood sample (see Appendix I).

• Some laboratories require an 8-hour fast before the test.

A venipuncture is performed and the sample is obtained in either a gray-topped or red-topped tube, depending on the laboratory performing the test. The sample should be handled gently to avoid hemolysis and transported promptly to the laboratory.

Nursing Care After the Procedure

Care and assessment after the procedure are the same as for any study involving the collection of a peripheral blood sample.

• If the client's diet was restricted before the test, the usual diet may be resumed.
• Abnormal levels: Note and report decreased levels. Assess hydration status for overhydration, I&O, osmolality, and sodium levels. Note and report increased levels and assess in relation to creatinine level. Assess electrolyte panel and for signs and symptoms of anemia, gastrointestinal bleeding, oliguria, confusion, and level of consciousness if urea nitrogen rises to greater than 20 to 50 mg/dL. Monitor urinary output every hour. Provide safety measures if consciousness is altered. Instruct as to restriction in fluid and dietary intake of protein (meat, fish, poultry).
• Critical values: Notify the physician immediately if levels are greater than 100 mg/dL.