A look at the laboratory testing of blood urea nitrogen bun

The blood urea nitrogen BUN test is done to measure the amount of nitrogen in your blood. The nitrogen is present in a chemical called urea.

A look at the laboratory testing of blood urea nitrogen bun

Definition Urea and creatinine are nitrogenous end products of metabolism. Urea is the primary metabolite derived from dietary protein and tissue protein turnover.

Creatinine is the product of muscle creatine catabolism. Both are relatively small molecules 60 and daltons, respectively that distribute throughout total body water.

In Europe, the whole urea molecule is assayed, whereas in the United States only the nitrogen component of urea the blood or serum urea nitrogen, i.

The range is wide because of normal variations due to protein intake, endogenous protein catabolism, state of hydration, hepatic urea synthesis, and renal urea excretion. The normal serum creatinine sCr varies with the subject's body muscle mass and with the technique used to measure it.

For the adult male, the normal range is 0. For the adult female, with her generally lower muscle mass, the normal range is 0. Technique Multiple methods for analysis of BUN and creatinine have evolved over the years. Most of those in current use are automated and give clinically reliable and reproducible results.

There are two general methods for the measurement of urea nitrogen. The diacetyl, or Fearon, reaction develops a yellow chromogen with urea, and this is quantified by photometry.

A look at the laboratory testing of blood urea nitrogen bun

It has been modified for use in autoanalyzers and generally gives relatively accurate results. It still has limited specificity, however, as illustrated by spurious elevations with sulfonylurea compounds, and by colorimetric interference from hemoglobin when whole blood is used.

In the more specific enzymatic methods, the enzyme urease converts urea to ammonia and carbonic acid. These products, which are proportional to the concentration of urea in the sample, are assayed in a variety of systems, some of which are automated.

One system checks the decrease in absorbance at mm when the ammonia reacts with alpha-ketoglutaric acid. The Astra system measures the rate of increase in conductivity of the solution in which urea is hydrolyzed.

Even though the test is now performed mostly on serum, the term BUN is still retained by convention. The specimen should not be collected in tubes containing sodium fluoride because the fluoride inhibits urease. Also chloral hydrate and guanethidine have been observed to increase BUN values.

This reaction is nonspecific and subject to interference from many noncreatinine chromogens, including acetone, acetoacetate, pyruvate, ascorbic acid, glucose, cephalosporins, barbiturates, and protein. It is also sensitive to pH and temperature changes.

One or another of the many modifications designed to nullify these sources of error is used in most clinical laboratories today.


For example, the recent kinetic-rate modification, which isolates the brief time interval during which only true creatinine contributes to total color formation, is the basis of the Astra modular system.

One of these, an automated dry-slide enzymatic method, measures ammonia generated when creatinine is hydrolyzed by creatinine iminohydrolase. Its simplicity, precision, and speed highly recommend it for routine use in the clinical laboratory.

Only 5-fluorocytosine interferes significantly with the test. Creatinine must be determined in plasma or serum and not whole blood because erythrocytes contain considerable amounts of noncreatinine chromogens. To minimize the conversion of creatine to creatinine, specimens must be as fresh as possible and maintained at pH 7 during storage.

Its primary source is dietary protein. In the hepatocyte, the amino acids are deaminated and transaminated. The resulting excess nitrogen feeds into the urea cycle to be incorporated into urea.

The protein moieties escaping absorption by the small bowel, plus recycled urea, are converted into ammonia by gut flora predominantly in the colon.

The ammonia diffuses through the portal circulation into the liver to enter the urea cycle Figure Modified from Raforth and Onstad, The amount of urea produced varies with substrate delivery to the liver and the adequacy of liver function. It is increased by a high-protein diet, by gastrointestinal bleeding based on plasma protein level of 7.

It is decreased by low-protein diet, malnutrition or starvation, and by impaired metabolic activity in the liver due to parenchymal liver disease or, rarely, to congenital deficiency of urea cycle enzymes. The normal subject on a 70 g protein diet produces about 12 g of urea each day. This newly synthesized urea distributes throughout total body water.

Some of it is recycled through the enterohepatic circulation. Usually, a small amount less than 0.The director of a clinical laboratory is usually a board-certified medical doctor, PhD scientist, or in some cases, a medical laboratory scientist.

BUN (Blood Urea Nitrogen) Urea is a waste product, formed in the liver and excreted in the urine at a relatively constant rate. BUN (blood urea nitrogen) is a measurement of the nitrogen part of urea, and is therefore an indirect measurement of the urea in the bloodstream.

Investigators have failed to show the usefulness of screening electrolyte—sodium, potassium, chloride and bicarbonate—blood urea nitrogen and glucose levels. In spite of this, we observed that that practice continues to be widely used at our university medical center.

Blood Urea Nitrogen (BUN) Your BUN reflects the amount of nitrogen that is present in your body in the form of a waste product called urea.

A look at the laboratory testing of blood urea nitrogen bun

BUN is used to determine if there is extra nitrogenous wastes in your blood stream, which should have been filtered out of your kidneys. Blood urea nitrogen (BUN) measures how well the kidneys are working.

Urea is a nitrogen-containing waste product created in the body through the metabolism and breakdown of proteins. When the kidneys are not functioning properly, there is an increase of BUN levels in the blood.

BUN and Creatinine - Clinical Methods - NCBI Bookshelf

Renal Function Markers - Kidney Disease. Laboratory Testing. Serum creatinine, blood urea nitrogen (BUN), and estimated glomerular filtration rate (EGFR) – use for initial diagnosis of acute or chronic disease Click on number for test-specific information in the ARUP Laboratory Test Directory.

Kidney Profile

Blood Urea Nitrogen: What it is & Why is Yours High (or low) - Metabolic Effect Metabolic Effect