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The glucose challenge is performed by giving 50 grams of a glucose drink and then drawing a blood sample one hour later and measuring the level of blood glucose present. A blood sugar level of less than 140 mg/dl (7.8 mmol/L) at one hour is considered normal. Women with a blood sugar level greater than 140 mg/dl (7.8 mmol/L) may have gestational diabetes during pregnancy, and require a follow-up test called a 3-hour oral glucose tolerance test (OGTT).

The OGTT requires dietary preparation. For three days prior to the test, 150 grams of carbohydrates should be consumed daily. Then a fasting period of 10 to 14 hours before the test is required; only water is allowed during this time. Because of the fasting component, the OGTT is usually performed first thing in the morning in a doctor’s office or outpatient lab. An initial blood sample is drawn to measure fasting blood glucose levels, and then the patient is given 100 grams of a glucose drink (the same type as that given in the glucose challenge) to consume. After that, blood samples are drawn every hour for 3 hours and blood glucose levels are measured.

Tests and Diagnosis For Gestational Diabetes

Urinary Glucose Testing:Women with GDM may have high glucose levels in their urine (glucosuria). Although dipstick testing is widely practiced, it performs poorly, and discontinuing routine dipstick testing has not been shown to cause underdiagnosis where universal screening is performed. Increased glomerular filtration rates during pregnancy contribute to some 50% of women having glucose in their urine on dipstick tests at some point during their pregnancy. The sensitivity of glucosuria for GDM in the first 2 trimesters is only around 10% and the positive predictive value is around 20%.

Oral glucose tolerance test:The OGTT should be done in the morning after an overnight fast of between 8 and 14 hours. During the three previous days the subject must have an unrestricted diet (containing at least 150 g carbohydrate per day) and unlimited physical activity. The subject should remain seated during the test and should not smoke throughout the test.The test involves drinking a solution containing a certain amount of glucose, and drawing blood to measure glucose levels at the start and on set time intervals thereafter.

The diagnostic criteria from the National Diabetes Data Group (NDDG) have been used most often, but some centers rely on the Carpenter and Coustan criteria, which set the cutoff for normal at lower values. Compared with the NDDG criteria, the Carpenter and Coustan criteria lead to a diagnosis of gestational diabetes in 54 percent more pregnant women, with an increased cost and no compelling evidence of improved perinatal outcomes.

Screening glucose challenge test:The screening glucose challenge test (sometimes called the O’Sullivan test) is performed between 24–28 weeks, and can be seen as a simplified version of the oral glucose tolerance test (OGTT). It involves drinking a solution containing 50 grams of glucose, and measuring blood levels 1 hour later.

If the cut-off point is set at 140 mg/dl (7.8 mmol/l), 80% of women with GDM will be detected. If this threshold for further testing is lowered to 130 mg/dl, 90% of GDM cases will be detected, but there will also be more women who will be subjected to a consequent OGTT unnecessarily.

Non-challenge blood glucose tests:When a plasma glucose level is found to be higher than 126 mg/dl (7.0 mmol/l) after fasting, or over 200 mg/dl (11.1 mmol/l) on any occasion, and if this is confirmed on a subsequent day, the diagnosis of GDM is made, and no further testing is required. These tests are typically performed at the first antenatal visit. They are patient-friendly and inexpensive, but have a lower test performance compared to the other tests, with moderate sensitivity, low specificity and high false positive rates.

Risk assessment for gestational diabetes mellitus should be undertaken at the first prenatal visit. Women with clinical characteristics consistent with a high risk of gestational diabetes mellitus (marked obesity, personal history of gestational diabetes mellitus, glycosuria, or a strong family history of diabetes) should undergo glucose testing (see below) as soon as feasible. If they are found not to have gestational diabetes mellitus at that initial screening, they should be retested between 24 and 28 weeks of gestation. Women of average risk should have testing undertaken at 24 to 28 weeks of gestation.

Every pregnant woman should be offered a screening test for gestational diabetes. Screening identifies apparently healthy people who may be at increased risk of a condition, such as diabetes. You can then be offered information, further tests and treatment to reduce the risk of complications from that condition.
Screening

You may be screened for gestational diabetes at your booking appointment. This is usually a long antenatal appointment with your midwife or GP, which takes place around weeks 8-12 of your pregnancy. See the Health A-Z topic about Antenatal appointments for more information about your care during pregnancy.

At this time, your GP or midwife will find out if you are at increased risk of gestational diabetes. They will do this by asking about any risk factors that may affect you, such as whether you have a family history of diabetes. See Gestational diabetes causes for a full list of the risk factors you will be asked about.

If any one of these risk factors applies to you, you will be offered a test for gestational diabetes.