To learn about test characteristics, read Gestational Diabetes Screening Tests.
The diagnostic test for gestational diabetes is called an Oral Glucose Tolerance Test (OGTT). The pregnant woman will drink juice with a dose of 75 g or 100 g of glucose (depending on the medical prescription). Then two blood tests will be taken one and two hours after drinking the liquid. This is to verify if the woman’s pancreas produces enough insulin to combat the sudden increase in glucose.
Usually, pregnant women take the OGTT test on an empty stomach while maintaining their regular diet and activities for the three days before the test. Note that caffeine naturally increases glucose.
The three-hour OGTT test will diagnose if a woman has gestational diabetes or not. If there are two measurements above normal levels, gestational diabetes will be diagnosed. Measurement examples:
- If glucose on an 8-hour empty stomach is above 7 mmol/litre
- If after two hours post-75 g glucose test the result is above 11.1 mmol/l
- If glucose, irrespective of meals, is above 11.1 mmol/l
On the other hand, if there is only one higher measurement, this is more glucose intolerance.
A blood glucose test is also possible. When a pregnant woman is offered a blood glucose test, they have to be taught how to prick their finger four times a day for one week, on waking and then one hour after every meal. The mother will note the measurements in a notebook, and then the health professional will check the results to ensure that they are between 3.5-5.2 mmol/l on waking and not above 7.1-7.7 mmol/l after meals.
This allows us to check a larger week-long sample to understand daily glucose fluctuations better.
To learn more about the degrees of diabetes, see Classifying Gestational Diabetes.