BUN-to-creatinine ratio
Encyclopedia
In medicine
, the BUN-to-creatinine ratio is the ratio
of two serum laboratory values, the blood urea nitrogen
(BUN) (mg/dL) and serum creatinine (mg/dL) (Cr). Outside the United States
, particularly in Canada
and Europe
, the truncated term urea
is used (though it is still the same blood chemical) and the units are different (mmol/L). The units of creatinine are also different (umol/L), and this value is termed the urea-to-creatinine ratio. The ratio may be used to determine the cause of acute kidney injury.
The principle behind this ratio is the fact that both urea (BUN) and creatinine are freely filtered by the glomerulus
, however urea reabsorbed by the tubules can be regulated (increased or decreased) whereas creatinine reabsorption remains the same (minimal reabsorption).
Serum Ratios
An elevated BUN:Cr due to a low or low-normal creatinine and a BUN within the reference range is unlikely to be of clinical significance.
from the gastrointestinal (GI) tract
in patients who do not present with overt vomiting of blood. In children, a BUN:Cr ratio of 30 or greater has a sensitivity of 68.8% and a specificity of 98% for upper gastrointestinal bleeding.
BUN level increases in upper GI bleeding because patients become prerenal, secondary to blood loss which decreases blood flow to the kidney.
A common misconception is that the ratio is elevated because of amino acid digestion. Since blood, which consists largely of the protein
hemoglobin
, is broken down by digestive enzyme
s of the upper GI tract into amino acids, which are then reabsorbed in the GI tract and broken down into urea. However, elevated BUN:Cr ratios are not observed when other high protein loads (e.g., steak) are consumed.
mass, elderly patients may have an elevated BUN:Cr at baseline.
Medicine
Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....
, the BUN-to-creatinine ratio is the ratio
Ratio
In mathematics, a ratio is a relationship between two numbers of the same kind , usually expressed as "a to b" or a:b, sometimes expressed arithmetically as a dimensionless quotient of the two which explicitly indicates how many times the first number contains the second In mathematics, a ratio is...
of two serum laboratory values, the blood urea nitrogen
Blood urea nitrogen
The blood urea nitrogen test is a measure of the amount of nitrogen in the blood in the form of urea, and a measurement of renal function. Urea is a by- product from metabolism of proteins by the liver and is removed from the blood by the kidneys.-Physiology:The liver produces urea in the urea...
(BUN) (mg/dL) and serum creatinine (mg/dL) (Cr). Outside the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, particularly in Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
and Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
, the truncated term urea
Urea
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....
is used (though it is still the same blood chemical) and the units are different (mmol/L). The units of creatinine are also different (umol/L), and this value is termed the urea-to-creatinine ratio. The ratio may be used to determine the cause of acute kidney injury.
The principle behind this ratio is the fact that both urea (BUN) and creatinine are freely filtered by the glomerulus
Glomerulus
A glomerulus is a capillary tuft that is involved in the first step of filtering blood to form urine.A glomerulus is surrounded by Bowman's capsule, the beginning component of nephrons in the vertebrate kidney. A glomerulus receives its blood supply from an afferent arteriole of the renal...
, however urea reabsorbed by the tubules can be regulated (increased or decreased) whereas creatinine reabsorption remains the same (minimal reabsorption).
Interpretation
Normal serum values| Test | SI units | US units |
|---|---|---|
| BUN (Urea) | 7-20 mg/dL | |
| Urea | 2.5-10.7 mmol/L | |
| Creatinine | 62-106 μmol/L | 0.7-1.2 mg/dL |
Serum Ratios
| BUN:Cr | Urea:Cr | Location | Mechanism |
|---|---|---|---|
| >20:1 | >100:1 | Prerenal (before the kidney Kidney The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and... ) |
BUN reabsorption is increased. BUN is disproportionately elevated relative to creatinine in serum. |
| 10-20:1 | 100-40:1 | Normal or Postrenal (after the kidney) | Normal range. Can also be postrenal disease. BUN reabsorption is within normal limits. |
| <10:1 | <40:1 | Intrarenal (within kidney) | Renal damage causes reduced reabsorption of BUN, therefore lowering the BUN:Cr ratio. |
An elevated BUN:Cr due to a low or low-normal creatinine and a BUN within the reference range is unlikely to be of clinical significance.
Acute Kidney Injury (previously termed Acute Renal Failure)
The ratio is predictive of prerenal injury when BUN:Cr exceeds 20 or when urea:Cr exceeds 0.1 and urea is greater than 10. In prerenal injury, urea increases disproportionately to creatinine due to enhanced proximal tubular reabsorption.Gastrointestinal bleeding
The ratio is useful for the diagnosis of bleedingUpper gastrointestinal bleeding
Upper gastrointestinal bleeding refers to hemorrhage in the upper gastrointestinal tract. The anatomic cut-off for upper GI bleeding is the ligament of Treitz, which connects the fourth portion of the duodenum to the diaphragm near the splenic flexure of the colon.Upper GI bleeds are considered...
from the gastrointestinal (GI) tract
Gastrointestinal tract
The human gastrointestinal tract refers to the stomach and intestine, and sometimes to all the structures from the mouth to the anus. ....
in patients who do not present with overt vomiting of blood. In children, a BUN:Cr ratio of 30 or greater has a sensitivity of 68.8% and a specificity of 98% for upper gastrointestinal bleeding.
BUN level increases in upper GI bleeding because patients become prerenal, secondary to blood loss which decreases blood flow to the kidney.
A common misconception is that the ratio is elevated because of amino acid digestion. Since blood, which consists largely of the protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...
, is broken down by digestive enzyme
Digestive enzyme
'Digestive enzymes' are enzymes that break down polymeric macromolecules into their smaller building blocks, in order to facilitate their absorption by the body. Digestive enzymes are found in the digestive tract of animals where they aid in the digestion of food as well as inside the cells,...
s of the upper GI tract into amino acids, which are then reabsorbed in the GI tract and broken down into urea. However, elevated BUN:Cr ratios are not observed when other high protein loads (e.g., steak) are consumed.
Advanced age
Because of decreased muscleMuscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
mass, elderly patients may have an elevated BUN:Cr at baseline.