Blood urea nitrogen

Blood urea nitrogen (BUN) is actually measured in serum rather than whole blood and can be assayed in urine. Urea can be measured biochemically or enzymatically (with the specific enzyme urease). Few substances interfere seriously with either method. Two screening methods for BUN, called Urograph and Azostix, are commercially available. Evaluations to date indicate that both of these methods are useful as emergency or office screening procedures to separate normal persons (BUN level <20 mg/100 ml [7 mmol/L]) from those with mild azotemia (20-50 mg/100 ml [7-18 mmol/L]) and those with considerable BUN elevation (>50 mg/100 ml [18 mmol/L]). If accurate quantitation is desired, one of the standard quantitative BUN procedures should be done.

As noted previously, urea is produced in the liver and excreted by the kidneys. When the kidneys are not able to clear urea sufficiently, urea accumulates in the blood. If reasonable liver function is assumed, measurement of urea (BUN) thus provides an estimate of renal function. Elevation of BUN levels is also known as azotemia. However, elevated BUN levels are not specific for intrinsic kidney disease. Elevated BUN levels may occur from excessive quantities of urea presented to the kidney; from decreased renal blood flow, which prevents adequate glomerular filtration; from intrinsic renal disease, which affects glomerular or tubular function; or from urinary obstruction, which results in back-pressure interference with urea removal. Therefore, in some types of azotemia the kidney is not structurally affected and the azotemia is transient. In other azotemic patients the primary cause is renal parenchymal damage, and whether the BUN elevation is reversible depends on whether the kidney is able to recover a sufficient degree of function. BUN levels may be decreased below expected levels in severe liver disease (insufficient manufacture) and sometimes in late pregnancy.

Following is a classification of azotemia based on etiology. It is subdivided into azotemia primarily due to increased urea or decreased blood flow (prerenal), intrinsic kidney disease, or postrenal obstruction.