What is an oGTT?
An oGTT tests how well the body processes the sugar (glucose) it receives. When sugar is ingested, it passes from the small intestine into the bloodstream, where it causes the glucose concentration to rise. The release of the hormone insulin from the pancreas causes the glucose to be channeled into liver, muscle and fat cells. This causes the blood sugar level to drop again. This is also known as glucose tolerance.
However, if glucose is only insufficiently introduced into the cells, this is referred to as impaired glucose tolerance or glucose intolerance. The glucose level in the blood remains elevated, which can be determined by a blood glucose measurement.
Which values are to be classified and how?
In an oGTT test, the glucose level in the blood is measured two hours after drinking the sugar solution:
- If the glucose value is then above 140 milligrams per deciliter (and the fasting blood sugar is below 126 mg/dl), there is impaired glucose tolerance (precursor to diabetes).
- If the measured value is 200 milligrams per deciliter or more, this indicates diabetes mellitus. The fasting blood sugar is then at least 126 mg/dl.
What values are normal during pregnancy?
If the oGTT value is too high after one hour and is 135 mg/dl or more, the 75 g oGTT is carried out under fasting conditions. In this test, the pregnant woman drinks a sugar solution containing 75 grams of sugar after eight hours of abstinence and the glucose level is measured again one hour and two hours later.
Gestational diabetes is present if, during this test:
- the fasting blood glucose (venous blood) is 92 mg/dl or higher and/or
- the blood glucose is 180 mg/dl or higher one hour after drinking the sugar solution and/or
- the blood glucose is 153 mg/dl or higher two hours after drinking the sugar solution.
It is sufficient for the diagnosis of “gestational diabetes” if one of these limits is exceeded.
When is an oGTT carried out?
The oGTT is carried out if there is a suspicion of impaired glucose tolerance, but if a measurement of the so-called fasting glucose (fasting blood sugar) does not provide any clearly disease-relevant results or if certain risk factors are present. These include
- Type 2 diabetes mellitus in first-degree relatives (such as parents)
- Overweight or physical inactivity
- Arterial high blood pressure (hypertension)
- Fat metabolism disorders
- Vascular diseases
- Proteins in the urine (albuminuria)
- Between the 24th and 28th week of pregnancy to rule out gestational diabetes (gestational diabetes)
How does the oGTT test work?
At least three days before the test, it is important to eat a carbohydrate-rich diet (150 to 250 grams of carbohydrates) so that the oGTT values are not distorted. This corresponds to a normal mixed diet. Eight to twelve hours before the oGTT, you must not eat, drink alcohol or sugared drinks or smoke.
A blood sample is first taken to determine the fasting glucose. You then drink a sweet test solution (75 grams of dextrose dissolved in 250 to 300 milliliters of water). After two hours, another blood sample will be taken to measure your blood sugar. During this time, you must not exercise or smoke.
There is a fundamental risk of the test being falsified. Under certain conditions, an oGTT is not meaningful:
- Acute infections and serious illnesses
- Stomach and duodenal ulcers
- Functional disorders of the liver
- Acute potassium and magnesium deficiency
- Three days before, during and three days after menstruation
- After stomach operations
It is also possible that drugs containing cortisone, antihypertensives (beta-blockers) and diuretics interfere with glucose tolerance. Your doctor will tell you which medications you can continue to take before the test.
What are the risks of an oGTT?
What do I have to consider after an oGTT?
Your doctor will discuss the results of the oGTT with you. If the oGTT values show impaired glucose tolerance, your blood sugar levels will be checked regularly and the risk factors for cardiovascular diseases, such as lipid metabolism disorders or high blood pressure, will be examined.
In addition, adjust your diet to your calorie requirements and, if possible, eat a high-fiber diet with plenty of fresh fruit and vegetables. If you are overweight, try to lose weight. Avoid ready-made products and alcohol. Regular exercise also helps to keep blood sugar levels stable, as the muscles then utilize more glucose.
Stopping smoking (quitting tobacco) is also highly recommended. It has a long-term positive effect on the development of diabetes mellitus and is also useful for prevention.
As a rule, a further oGTT is carried out after three to six months.
