BUN

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Normal: 8 to 20 mg/dL (2.9 to 7.1 mmol/L)

Physiology:

BUN is the concentration of nitrogen in the serum. Urea is an end product of protein metabolism normally excreted via the kidneys. It reflects recent intake.

Low BUN: less than 8 mg/dL

Causes:

•	BUN may be low in patients who are malnourished or have profound liver damage.

•	Fluid overload may initially dilute BUN causing a falsely low concentration.

Assessment: Check the BUN to creatinine ratio, the normal ratio is 10:1.

Correction: Check for underlying causes : fluid overload, low protein intake, inability to metabolize protein due to liver failure.

TPN relevance: Assists the clinician in determining the fluid and electrolyte status of the patient.

High BUN (Azotemia): levels above 20 mg/dL

Causes:

•	Amino acid administration

•	Upper GI bleeding

•	Medications: corticosteroids, tetracyclines and other drugs with antianabolic effects.

Assessment: Check the BUN to creatinine ratio, the normal ratio is 10:1. When a patient becomes dehydrated,the BUN to creatinine ratio is greater than 10. The ratio is also high when there is protein catabolism and high protein intake.

Correction: Check underlying causes: fluid status of the patient/dehydration, factors influencing protein catabolism and reduced renal function. Check for GI bleeding.

TPN relevance: Assist the clinician in determining the fluid and electrolyte status of the patient.