Oral Glucose Tolerance Test

The oral glucose tolerance test (OGTT) measures the body's ability to use a type of sugar, called glucose, that is the
body's main source of energy. An OGTT can be used to diagnose prediabetes and diabetes. An OGTT is most
commonly done to check for diabetes that occurs with pregnancy (gestational diabetes).

Why It Is Done

The oral glucose tolerance test (OGTT) is done to:

 Check pregnant women for gestational diabetes. When done for this purpose, the test is called a glucose
challenge screening test, and it is usually done during the 24th to the 28th week of pregnancy. You have an
increased chance of developing gestational diabetes if you:
o Have had gestational diabetes during a previous pregnancy.
o Have previously given birth to a baby who weighed more than 9lb.
o Are younger than age 25 and were overweight before getting pregnant.
 Diagnose gestational diabetes if other blood glucose measurements are high.
 Screen women who have polycystic ovary syndrome (PCOS) for diabetes.
 Diagnose prediabetes and diabetes.

How To Prepare

Glucose challenge screening test for gestational diabetes

No preparation is usually needed for the screening test done during pregnancy. This test can be done at any time of
the day, so, you do not need to limit food or fluids before the test.

Glucose tolerance diagnostic test

To prepare for the glucose tolerance diagnostic test:

 Eat a balanced diet that contains at least 150 to 200 grams (g) of carbohydrate per day for 3 days before the
test. Fruits, breads, cereals, grains, rice, crackers, and starchy vegetables such as potatoes, beans, and corn
are good sources of carbohydrate.
 Do not eat, drink, smoke, or exercise strenuously for at least 8 hours before your first blood sample is
taken.
 Tell your doctor about all prescription and nonprescription medicines you are taking. You may be
instructed to stop taking certain medicines before the test.

The glucose tolerance diagnostic test may take up to 4 hours. Since activity can interfere with test results, you will
be asked to sit quietly during the entire test. Do not eat during the test. You may drink only water during this time.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, or how it will be done. To
help you understand the importance of this test, fill out the medical test information form (What is a PDF
document?).

How It Is Done

Glucose challenge screening test for gestational diabetes

 You will be asked to drink a sweet liquid containing glucose. It is best to drink the liquid quickly.
  A blood sample will be collected 1 hour after you drink the glucose. Normally, blood glucose levels peak
within an hour and then begin to drop.

The Hemoglobin A1c (HbA1c) Test for Diabetes

Importance of Hemoglobin A1c Test

The hemoglobin A1c test -- also called HbA1c, glycated hemoglobin test, or glycohemoglobin -- is an important
blood test used to determine how well your diabetes is being controlled. Hemoglobin A1c provides an average of
your blood sugar control over a six to 12 week period and is used in conjunction with home blood sugar monitoring
to make adjustments in your diabetes medicines.

Hemoglobin is a substance within red blood cells that carries oxygen throughout your body. When your diabetes is
not controlled (meaning that your blood sugar is too high), sugar builds up in your blood and combines with your
hemoglobin, becoming "glycated." Therefore, the average amount of sugar in your blood can be determined by
measuring a hemoglobin A1c level. If your glucose levels have been high over recent weeks, your hemoglobin A1c
test will be higher. The amount of hemoglobin A1c will reflect the last several weeks of blood sugar levels, typically
encompassing a period of 120 days.

What's a Normal Hemoglobin A1c Test?

For people without diabetes, the normal range for the hemoglobin A1c test is between 4% and 6%. Because studies
have repeatedly shown that out-of-control diabetes results in complications from the disease, the goal for people
with diabetes is an hemoglobin A1c less than 7%. The higher the hemoglobin A1c, the higher the risks of
developing complications related to diabetes.

People with diabetes should have this test every three months to determine whether their blood sugars have reached
the target level of control. Those who have their diabetes under good control may be able to wait longer between the
blood tests, but experts recommend checking at least 2 times a year.

Patients with diseases affecting hemoglobin such as anemia may get abnormal results with this test. Other
abnormalities that can affect the results of the hemoglobin A1c include supplements such as vitamins C and E and
high cholesterol levels. Kidney disease and liver disease may also affect the result of the hemoglobin A1c test.

IVGTT

The intravenous glucose tolerance test (IVGTT) is used to evaluate islet function in vivo. It does not have
greater diagnostic value than the OGTT (oral glucose tolerance test) in the diagnosis of diabetes mellitus
so is used only in research. It measures the early insulin response following an intravenous glucose load,
and is useful in transplantation studies and in prediabetic individuals with elevated islet cell antibodies and
other autoimmune markers.

The classic minimal model of glucose kinetics (Bergman RN, Finegood DT, Ader M. Assessment of
insulin sensitivity in vivo. Endocrine Reviews 1985; 6:45-86.) is widely used to investigate glucose
metabolism in vivo in physiological, pathological and epidemiological studies. The standard IVGTT
consists in injecting, say at time 0, glucose over a period of 30-60 seconds and measuring in plasma the
resultant glucose and insulin concentrations. The sampling schedule of the standard IVGTT usually
consists of 3 pretest samples taken at -15, -5 and 0 (immediately before the injection) and 25 test
samples taken at 2, 3, 4, 5, 6, 8, 10, 12, 15, 18, 20, 25, 30, 35, 40, 60, 70, 80, 100, 120, 140, 160, 180,
210, 240 minutes.
The following figure shows results of the IVGTT of a normal (nondiabetic) as the red line with round
points. (Click for a larger view.) The yellow and brown lines show abnormal IVGTT's obtained in one of
our experiments.

K Value

The results of an IVGTT are reported as a K Value, a reflection of the time required for glucose to clear
the circulation.

K = 0.69/T1/2 X 100, where T1/2 is the time required for plasma glucose to reach one half of the calculated
zero-time concentration. The normal value for K is 1.2 or greater.

Postprandial

Postprandial (noun form: postprandium) means after eating a meal[1] while preprandial is before a meal

This term is used in many contexts but also in relation to blood sugar (or blood glucose) levels, which are normally
measured 2 hours after and before eating in a postprandial glucose test. For example, the American Diabetes
Association recommends a postprandial glucose level under 180 mg/dl and a preprandial plasma glucose between
90-130 mg/dl. [2] However this is not the only context in which this term is used.

 Postprandial hyperglycemia (PPHG) is high blood sugar following a meal. It can be evaluated in a
postprandial glucose test.
 Postprandial dip is a mild decrease in blood sugar after eating a big meal, possibly leading to drowsiness
 Postprandial hypotension is a drastic decline in blood pressure which happens after eating [3]
 Postprandial regurgitation is a unique symptom of rumination syndrome

Postprandial thermogenesis increases basal metabolic rate to different degree depending on food constituents.
  Postprandial thermogenesis is heat production due to metabolism after a meal, temporarily increasing the
basal metabolic rate.

[edit] Processes in the postprandium

In the postprandium, there is digestion of food in the gastrointestinal tract, followed by uptake and various metabolic
processes, mainly anabolic ones (building organic molecules from smaller units). The postprandium is characterized
by an increased activity of the parasympathetic nervous system, putting the body in a state of "rest and digest".