During Postprandial Glucose study, a meal acts as a glucose challenge to the body’s metabolism. Insulin is normally secreted immediately after a meal in response to the elevated blood glucose level, causing the level to return to the premeal range within 2 hours. In patients with diabetes the glucose level usually is still elevated 2 hours after the meal. The PPG is an easily performed screening test for DM. If the results are greater than 140 and less than 200 mg/dL, a glucose tolerance test may be performed to confirm the diagnosis. If the 2-hour PPG is greater than 200 mg/dL, the diagnosis of DM is confirmed. Also, a glucose tolerance or glycosylated hemoglobin test can be performed to corroborate and better evaluate the disease.
The 2-hour PPG test is a measurement of the amount of glucose in the patient’s blood 2 hours after a meal is ingested (postprandial). It is used to diagnose diabetes mellitus (DM). When the 2 Hour Postprandial Glucose test is performed, a fasting blood glucose test is performed before the patient takes the meal. The patient should start a 12 hours of fasting before performing the test. The fasting glucose reading will be used as a baseline glucose level to give an idea if the patient glucose level was already high or not before the test.
The meal should contain at least 75 grams of carbohydrates and the patient should eat the whole meal. After taking the meal, the patient should not eat or smoke until the blood sample is drawn 2 hours later since such activities would increase glucose levels in the blood. Patients also are supposed to relax and avoid exhausting activities or exercising during the 2 hours waiting period because the body burns its fat deposit when it comes to exercising which increases glucose levels in blood.
The 1-hour glucose screen is used to detect gestational DM, which is the most common medical complication of pregnancy. Gestational diabetes is a carbohydrate intolerance first recognized during pregnancy and affects 3% to 8% of pregnant women, with up to half of these women developing overt diabetes later in life. The detection and treatment of gestational diabetes may reduce the risk for several adverse perinatal outcomes (e.g., excessive fetal growth and birth trauma, fetal death, neonatal morbidity).
Screening for gestational diabetes is performed with a 50-g oral glucose load followed by a glucose level determination 1 hour later. This is called the O’sullivan test. Screening is done between 24 and 28 weeks of gestation. However, patients with risk factors such as a previous history of gestational diabetes may benefit from earlier screening. Patients whose serum glucose level equals or exceeds 140 mg/dL should be evaluated by a 3-hour glucose tolerance test.
Causes of Postprandial Glucose False results:
- Smoking during the testing period may increase the blood glucose level.
- Stress can increase glucose levels through the catecholamine effect of increasing serum glucose.
- If the patient eats a small snack or eats candy during the 2-hour interval, glucose levels will be falsely elevated.
- If the patient is not able to eat the entire test meal or vomits some or all of the meal, levels will be falsely decreased.
Normal Postprandial Glucose Levels
As people grow in age their Insulin response to what they eat slows down. The normal Postprandial Glucose Levels for healthy persons who are younger than 50 years of age are lower than 140 mg/dl (Milligram/Deciliter). A healthy person whose age is between 50 and 60 years should have a Postprandial Glucose Level less than 150 mg/dL. After 60 years of age, a person with no signs of high blood sugar would have a Postprandial Glucose Level less thank 160 mg/dL.
Causes of High Postprandial Glucose Levels
Diabetes mellitus (DM): This disease is defined by glucose intolerance and hyperglycemia. A discussion of the many possible etiologies is beyond the scope of this manual.
Gestational diabetes mellitus (DM): This disease is defined by glucose intolerance and hyperglycemia during pregnancy.
Malnutrition: Malnourished patients have very poor glucose tolerance when they start to eat. The pathophysiology and theories of this observation are not well defined and are multiple.
Hyperthyroidism: Thyroid hormone is an ancillary hormone that affects glucose metabolism and acts to increase glucose levels.
Acute stress response: Severe stress, including infection, burns, and surgery, stimulates catecholamine release. This in turn stimulates glucagon secretion, which causes hyperglycemia.
Cushing syndrome: Blood cortisol levels are high. This in turn causes hyperglycemia.
Pheochromocytoma: Catecholamine stimulates glucagon secretion, which causes hyperglycemia.
Chronic renal failure: Glucagon is metabolized by the kidney. With loss of kidney function, glucagon and glucose levels rise.
Glucagonoma: Glucagon is autonomously secreted, causing hyperglycemia.
Diuretic therapy: Certain diuretics cause hyperglycemia.
Corticosteroid therapy: Cortisol causes hyperglycemia.
Acromegaly: Growth hormone stimulates glucagon, which causes hyperglycemia.
Extensive liver disease: Most glucose metabolism occurs in the liver. With decreased function of the liver, glucose levels decrease.
Causes of Low Postprandial Glucose Levels
Insulinoma: Insulin is autonomously produced without regard to biofeedback mechanisms.
Hypothyroidism: Thyroid hormone affects glucose metabolism. With diminished levels of this hormone, glucose levels fall.
Addison disease: Cortisol affects glucose metabolism. With diminished levels of this hormone, glucose levels fall.
Insulin overdose: This is the most common cause of hypoglycemia. Insulin is administered at too high of a dose (especially in brittle diabetes), and glucose levels fall.
Malabsorption or maldigestion: The test meal is not absorbed and glucose levels do not increase.