Blood Glucose and Related Blood Sugar Tests

Glucose, a simple sugar, is the main blood carbohydrate and a major source of energy for all cells. The fasting blood sugar (FBS), postprandial blood sugar (PPBS), and the glucose tolerance test (GTT), or standard oral glucose tolerance test (SOGTT), are three of the most frequently performed blood sugar tests and are used to determine the level of glucose in the blood. Variations in blood glucose levels are broadly categorized as hyperglycemia, or increased blood sugar levels, and hypoglycemia, or decreased blood sugar levels.

Related blood sugar tests measure the body's ability to produce insulin and glucose. Insulin and glucose production can be monitored by measuring blood levels of C-peptide, the residue of insulin formation; glucagon, a hormone that stimulates the production of glucose; and insulin, the hormone responsible for glucose metabolism. Table 2-2 identifies each blood glucose test and related diagnostic applications.

Table 2-2. Blood Glucose and Related Blood Sugar Tests Test Diagnostic Application

Fasting blood sugar (FBS) Diabetes screening

Postprandial blood sugar (PPBS) Confirms diabetes

Glucose tolerance test (GTT) Rule out diabetes; confirm hypoglycemia

C-peptide Secretory function of pancreatic beta cells;

inappropriate insulin injection

Glucagon Assess pancreatic conditions and disorders

Insulin Diabetes; secretory function of pancreatic alpha cells

Fasting Blood Sugar (FBS)

The fasting blood sugar test measures the plasma level of glucose. Results are reported as the number of milligrams per deciliter (mg/dl) of blood. The test is performed to detect any disorder of glucose metabolism, primarily diabetes, and is also used to assess the management of diabetes. As the name implies, the client must refrain from eating approximately four to twelve hours prior to the test. If the client is an insulin-dependent diabetic, both food and insulin can be withheld until the blood specimen is drawn.

Normal Range

Adults 70-115 mg/dl

Children 60-110 mg/dl Newborns 30-80 mg/dl

Variations from Normal. An increase in blood glucose, hyperglycemia, usually indicates diabetes. Myocardial infarction, meningitis, or encephalitis, all of which produce acute stress in bodily processes, may also cause an elevated blood glucose level. Other conditions associated with hyperglycemia include an increased secretion of glucocorticoids from the adrenal glands as seen in Cushing's disease, pituitary and pancreatic adenomas, pancreatitis, hyperthyroidism, and chronic illness or inactivity.

Hyperglycemia is sometimes seen during pregnancy and is called gestational diabetes. The condition is usually diagnosed during the latter half of the pregnancy and is caused by an increased secretion of the placental hormone lactogen. Lactogen can inhibit the action of insulin, thereby increasing the blood glucose level. Gestational diabetes presents a risk to the fetus and mother and must be closely monitored throughout the pregnancy.

Hypoglycemia, a decrease in blood glucose, is often caused by an overdose of insulin or skipping meals. Other causes of hypoglycemia include pancreatic islet cell malignancy, severe liver damage, hypothyroidism, Cortisol deficiency, and pituitary hormone deficiency.

Interfering Circumstances. Many drugs can interfere with fasting blood sugar results. Steroids, particularly prednisone, and diuretics can significantly alter test results. Anesthesia, stress, and obesity may also affect blood glucose levels.

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