Urea Nitrogen

High BUN occurs in chronic glomerulonephritis, pyelonephritis and other causes of chronic renal disease; with acute renal failure, decreased renal perfusion (prerenal azotemia) as in shock. With urinary tract obstruction BUN increases (postrenal azotemia), for example as caused by neoplastic infiltration of the ureters, hyperplasia or carcinoma of the prostate. BUN is useful to follow hemodialysis and other therapy. “Uremia” was defined by Luke as an expression of a constellation of signs and symptoms in patients with severe azotemia secondary to acute or chronic renal failure.¹ Causes of increased BUN include severe congestive heart failure, catabolism, tetracyclines with diuretic use, hyperalimentation, ketoacidosis, and dehydration as in diabetes mellitus, but even moderate dehydration can cause BUN to increase. Corticosteroids tend to increase BUN by causing protein catabolism. Bleeding from the gastrointestinal tract is an important cause of high urea nitrogen, commonly accompanied by elevation of BUN:creatinine ratio. Nephrotoxic drugs must be considered.

Borderline high values may occur after recent ingestion of high protein meal and muscle wasting may cause an elevation as well.

With creatinine, BUN is used to monitor patients on dialysis.

Low BUN occurs in normal pregnancy, decreased protein intake, with intravenous fluids, with some antibiotics, and in some but not all instances of liver disease.

As described by DeCaux et al in 1980, in the syndrome of inappropriate secretion of antidiuretic hormone (SIADH): findings include hyponatremia with serum or plasma Na⁺ ≤128 mmol/L, hypo-osmolality (<260 mOsm/kg with urine osmolality >300 mOsm/kg) with low BUN. Such findings occur in situations in which patients are overhydrated. Clinical findings included absence of edema or evidence of heart, liver, thyroid, renal or adrenal disease.² Hypouricemia, with uric acid levels in 16 of 17 patients <4 mg/dL, is reported with the syndrome of inappropriate secretion of antidiuretic hormone.³ (SIADH can be seen with higher serum sodiums and higher osmolalities. Urine osmolality is greater than serum osmolality in SIADH. DeCaux in 1982 presented criteria modified from the 1980 paper.⁴)

Osmolality (mOsm/kg H₂O) is calculated as follows:

Osmolality = [Na⁺ (mmol/L)] x [2 + glucose (mg/dL)] / 18 + [BUN (mg/dL) / 2.8]