Glossary B

bilirubin, total: Bilirubin is a yellow breakdown product of hemoglobin, the oxygen-carrying molecule in red blood cells (RBCs). Bilirubin levels may be elevated in people with liver disease or a blocked bile duct, as seen with gallstones. The total bilirubin value includes both conjugated and unconjugated bilirubin. High unconjugated bilirubin levels are most commonly seen in newborns. Elevated conjugated bilirubin levels may occur when any portion of the biliary tree becomes blocked or abnormally permeable, retarding the passage of bilirubin and other bile components normally to the intestinal tract.
blood urea nitrogen (bun): This test measures the amount of urea nitrogen in the blood. Urea is the major breakdown product of bodily protein and contains nitrogen. Urea and other nitrogen-rich waste products are normally eliminated from the bloodstream by the kidneys, so an increased blood urea nitrogen (BUN) level may indicate impaired renal (kidney) function. However, many other conditions other than renal disease can cause BUN alterations. An elevated BUN level may also be caused by congestive heart failure, dehydration, shock, hemorrhage into the gastrointestinal tract, acute myocardial infarction, stress, or excessive protein intake or protein breakdown. A decreased level may be seen in liver failure, malnutrition, anabolic steroid use, or impaired nutrient absorption. In patients with liver disease, the BUN level may be low even if the kidneys are normal.
blood urea nitrogen/creatinine ratio: Unlike urea, blood creatinine is relatively insensitive to body hydration and the presence of blood or meat in the intestinal tract; therefore, the blood urea nitrogen (BUN) to creatinine ratio may help to distinguish impaired kidney function due to dehydration from other kinds of kidney damage. The ratio of BUN:creatinine is normally 10:1, but can increase to 20:1 or even higher with dehydration. An increased BUN:creatinine ratio may also be due to certain types of kidney disease, breakdown of blood in the intestinal tract, increased dietary protein, or any clinical circumstance resulting in reduced blood flow to the kidneys (such as heart failure or renal artery disease). The BUN:creatinine ratio is decreased in other renal conditions, liver disease, malnutrition and sickle cell anemia.
bordetella pertussis antibody (b. pertussis Ab): Bordetella pertussis (B. pertussis) antibody is evidence of recent infection with, exposure to, or vaccination with B. pertussis, the bacterium causing whooping cough.
brain-derived neurotrophic factor(bdnf): Brain-derived neurotrophic factor (BDNF) is a protein found in the central nervous system, which is important to the survival of existing brain cells and the growth of new brain cells. BDNF levels may be abnormally low in certain nervous system condition such as depression and Alzheimer’s disease, as well as other mood disorders. Research has shown that persistent exposure to stress and the stress hormone corticosterone decreases the expression of BDNF and leads to atrophy of part of the brain, specifically the hippocampus. Physical activity has been reported to increase BDNF expression.
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