Oral Glucose Tolerance Test (OGTT)

Oral Glucose Tolerance Test (OGTT)

The oral glucose tolerance test shows how much blood glucose rises after drinking a defined sugar solution. The test helps to identify people with increased risk of diabetes or manifest type 2 diabetes. It uses commonly during pregnancy for diagnosing gestational diabetes mellitus (GDM).

What is an OGTT?

• An oral glucose tolerance test (OGTT) is designed to measure how well the body can process a large amount of sugar. If the blood glucose exceeds certain values during the test, it may be that the sugar is not being absorbed sufficiently into the body’s cells. It is then possible that diabetes mellitus or gestational diabetes is present.
• When sugar ingests, it passes from the small intestine into the blood and causes the blood glucose concentration to rise.
• The release of the hormone insulin from the pancreas sends the glucose into liver, muscle, and fat cells.
• It causes the blood glucose level to drop again.
• It is also known as glucose tolerance.
• However, if the glucose introduces only inadequately into the cells, this is referred to as impaired glucose tolerance or glucose intolerance.
• The blood glucose level remains elevated, which can determine by measuring blood glucose.

Why is an OGTT done? (What are the purposes of the test?)

The OGTT performs when impaired glucose tolerance is suspected, but when a measurement of so-called fasting glucose (fasting blood glucose) does not provide disease-relevant results or when certain risk factors are present. These include:

• Overweight or physical inactivity
• Type 2 diabetes mellitus in first-degree relatives (such as parents).
• Dyslipidemia
• Arterial high blood pressure (hypertension)
• Vascular diseases
• Proteins in the urine (albuminuria)
• Between the 24th and 28th week of pregnancy to exclude gestational diabetes (pregnancy diabetes)

The more risk factors are present, the sooner a possible glucose intolerance should be clarified with an OGTT.

When is the test not reliable?

Under certain conditions, an OGTT is not meaningful:

• Liver dysfunction
• After stomach surgery
• Acute infections and severe illnesses
• Gastric and duodenal ulcers
• Acute potassium and magnesium deficiency
• Three days before, during, and three days after menstruation

• In addition, medications containing cortisone, antihypertensives (beta-blockers), and diuretics can interfere with glucose tolerance. Your doctor will tell you which medications you may continue to take before the exam.

How is the test performed?

Usually, the patient drinks 250 ml of water with 75 grams of glucose after not eating for about ten hours. Before drinking the sugar solution and after one and two hours, the health care provider draws venous blood and determines the blood sugar in the plasma.

 Before the test

• At least three days before the test, you should eat a diet rich in carbohydrates (150 to 250 grams of carbohydrates per day so that the OGTT values will not be false.
• It corresponds to a usual mixed diet.
• Eight to twelve hours before the OGTT, you must not eat anything, consume alcohol or sugared drinks, or smoke (water is an exception).

During the test

Type-2 Diabetes

• A blood sample will take first from the vein of your arm to determine fasting glucose.
• Immediately after taking the blood sample, you will drink 75 grams of glucose dissolved in 250 to 300 milliliters of water within five minutes.
• In children, the amount of glucose of 1.75 g/kg of body weight up to 75 g being the upper limit is used.
• After two hours, blood is drawn again for another glucose test.
• It should note that no eating, drinking, or smoking is allowed during the entire duration of the test.

Gestational Diabetes

• The sugar test is offered in the 6th or 7th month (or 24 – 28 Weeks) of pregnancy.
• You will get two tests to diagnose.

First Pre-Test:

• You will first do a pre-test where you drink a glass of water with 50 grams of sugar.
• You do not have to be fasting for this pre-test.
• After one hour, blood will draw from a vein in your arm and, the blood sugar will determine.
• If the value is below 7.5 millimoles per liter (mmol/l, which corresponds to 135 mg/dl), the result is unremarkable, and the test is over.
• If an elevated value is found in the pre-test, this is not yet a diagnosis.

Second Test:

• The pre-test serves to identify women who are then offered a second, decisive sugar test.
• This second test is more complex. For this “diagnostic test”, the pregnant woman must be fasting, i.e., not have eaten, or drunk anything for at least twelve hours, only water is allowed.
• The test begins by drawing blood from the woman while she is fasting.
• Only then does she drink a sugar solution containing 75 grams of glucose.
• After one and after two hours, blood is again taken from a vein in the arm.

After the test

• After the glucose tolerance test, you will suggest eating immediately to prevent low blood sugar (hypoglycemia), and you can return to your usual activities immediately.

What do these OGTT results mean? (Interpreting test results)

Type 2 Diabetes

Normal Results for Type 2 Diabetes

• Fasting glucose level 60 to 100 mg/dL
• One-hour glucose level less than 200 mg/dL
• Two-hour glucose level less than 140 mg/dL

Impaired Results for Type 2 Diabetes

• Fasting glucose level: 100 to 125 mg/dL
• Two-hour glucose level 140 to 200 mg/dL

Abnormal (Diagnostic) Results for Type 2 Diabetes

• Fasting glucose level greater than 126 mg/dL
• Two-hour glucose level greater than 200 mg/dL

During Pregnancy

Normal Results for Gestational Diabetes

• Fasting glucose level less than 90 mg/dL
• One-hour glucose level less than 130 to 140 mg/dL
• Two-hour glucose level less than 120 mg/dL

Abnormal Results for Gestational Diabetes

If one of the following three blood glucose values is reached or exceeded, the diagnosis of “gestational diabetes” is made:

• Fasting: greater than 95 mg/dL (5,3 mmol/l)
• After one hour: greater than 180 mg/dL (10,0 mmol/l
• After two hours: greater than 153 mg/dL (8,5 mmol/l)

References

1. Glucose Tolerance Test – StatPearls – NCBI Bookshelf (nih.gov)
2. Glucose tolerance test – Mayo Clinic
3. RACGP – Oral glucose tolerance testing
4. Function tests | Labor Krone
5. Glucose Tolerance Test (diabetes.co.uk)
6. Glucose tolerance test – non-pregnant: MedlinePlus Medical Encyclopedia
7. Oral Glucose Tolerance Test for Gestational & Type 2 Diabetes (webmd.com)
8. Oral Glucose Tolerance Test: For Pregnant, Preparation & Results (medicinenet.com)