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Subject: Blood Urea Nitrogen (BUN)
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Protein and nucleic acid catabolism results in the formation of urea and ammonia. Urea is synthesized mainly in the liver, and >90% is excreted through the kidneys.
Normal range: 7–23 mg/dL.
Most widely used screening test for the evaluation of kidney function.
Along with the serum creatinine, BUN levels aid in the differential diagnosis of prerenal, renal, and postrenal hyperuremia.
Diagnosis of renal insufficiency: filtered freely in the glomerulus; ≤50% is reabsorbed.
Assessment of glomerular function: A BUN of 10–20 mg/dL almost always indicates normal glomerular function.
In chronic renal disease, BUN correlates better with symptoms of uremia than does serum creatinine.
Provides evidence of hemorrhage into the upper GI tract.
Assessment of patients requiring nutritional support for excess catabolism, for example, burns, cancer.
Impaired kidney function: A BUN of 50–150 mg/dL implies serious impairment of renal function. A markedly increased BUN (150–250 mg/dL) is virtually conclusive evidence of severely impaired glomerular function.
Prerenal azotemia—any cause of reduced renal blood flow:
Salt and water depletion (vomiting, diarrhea, diuresis, sweating)
Postrenal azotemia—any obstruction of urinary tract (increased BUN-to-creatinine ratio).
Increased protein catabolism (serum creatinine remains normal):
Hemorrhage into the GI tract
Diuresis (e.g., with overhydration, often associated with low protein catabolism).
Severe liver damage (e.g., drugs, poisoning, hepatitis). A low BUN of 6–8 mg/dL is frequently associated with states of overhydration or liver disease.
Increased utilization of protein for synthesis (e.g., late pregnancy, infancy, acromegaly, malnutrition, anabolic hormones)
Diet (e.g., low-protein and high-carbohydrate, IV feedings only, impaired absorption [celiac disease], malnutrition)
Nephrotic syndrome (some patients)
Inherited hyperammonemias (urea is virtually absent in blood)
Urea levels increase with age and protein content of the diet.
Corticosteroids, tetracyclines, and drugs causing nephrotoxicity frequently increase BUN.
The presence of ammonium ions in anticoagulants may produce falsely elevated results.