Blood glucose determination is one of the most common tests conducted in the clinical chemistry laboratory. Glucose is a monosaccharide formed from the degradation of carbohydrates in the diet. Carbohydrate absorption occurs in the small intestine, where monosaccharides, or single-sugar units of carbohydrates, are consumed. The liver converts nonglucose monosaccharides such as galactose and fructose to glucose.
A variety of hormones regulate the blood glucose level at any given time. Insulin, a hormone secreted by the pancreas after a meal, responds to elevated glucose levels by facilitating glucose entry into cells. Under the action of insulin, glucose can also be converted to protein and fat (lipogenesis), with the latter being retained as fat (adipose) tissue. Most body cells have small glycogen stores, but the liver and skeletal muscles have greater glycogen stores. Glycogen accounts for 10% of the liver’s overall weight.
Depending on the needs of the cell, glucose can undergo anaerobic or aerobic metabolism to produce energy in the form of adenosine triphosphate (ATP). The hormones glucagon, cortisol, and thyroxine (T4) induce gluconeogenesis, or the synthesis of glucose from lactate or amino acids. Alternatively, glucose can be converted biochemically to and processed as glycogen (glycogenesis).
Glucose is the main source of energy for the vast majority of body cells. Insulin controls blood glucose concentrations by facilitating its entrance into the cell, which is followed by a variety of metabolic fates such as glycolysis, glycogenesis, lipogenesis, and protein synthesis. Several hormones have the effect of keeping blood glucose levels stable when fasting. These hormones have different cellular effects, but they all raise blood glucose levels in order to counteract the action of insulin. Glucagon, secreted by the pancreatic alpha (α) cells, is the major hormone that opposes the action of insulin, increasing blood glucose by causing glycogen breakdown (glycogenolysis) by the liver. A variety of other hormones, some of which are secondary to pituitary hormone release, facilitate glycogenolysis, or the breakdown of glycogen to form glucose. After a carbohydrate-rich meal, plasma glucose levels rise quickly and return to normal 1½ to 2 hours later (postprandial level). The natural glucose metabolism is disrupted by a variety of diseases. Diabetes is the most common cause of an increase in blood glucose, or hyperglycemia.
Hypoglycemia, known as a blood glucose level of less than 50mg/dL, can be fatal. An overdose of insulin is one cause of hypoglycemia in diabetic patients. A variety of hormones are secreted by the body that raise blood glucose levels, but only insulin lowers blood sugar.