| CLINICAL BIOCHEMISTRY DEPARTMENT |
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ORAL GLUCOSE TOLERANCE TEST (OBSTETRIC PATIENTS)
INTRODUCTION
Following a standard dose of glucose, plasma glucose is monitored at regular intervals in order to measure glucose tolerance under defined conditions. The results may be influenced by a number of factors and it is important that patient preparation instructions are carefully followed.
INDICATIONS
Testing for gestational diabetes is usually done between 24 and 28 weeks of gestation.
Pregnant women who are low risk do not usually require screening by GTT for Gestational Diabetes. Low risk patients are <25 years of age, normal body weight, have no family history of diabetes, have no history of abnormal glucose and no history of poor obstetric outcome.
Individuals at high risk for gestational diabetes include older women, those with previous history of glucose intolerance, those with a history of large for gestational age babies, women from certain high–risk ethnic groups, and any pregnant woman who has elevated fasting, or casual, blood glucose levels. It may be appropriate to screen pregnant women belonging to high–risk populations during the first trimester of pregnancy in order to detect previously undiagnosed diabetes mellitus.
PRECAUTIONS
Please refer to the Trust’s phlebotomy procedure for sample collection precautions.
PATIENT PREPARATION
Various drugs can influence glucose tolerance and should be stopped for a period (ideally about 5 times the half-life of the drug) before glucose tolerance testing. Drugs which decrease glucose tolerance include corticosteroids, thiazides and sympathomimetic agents. Recent infection may also impair glucose tolerance. The presence of factors that influence interpretation of the results of the test must be recorded (e.g. medications, inactivity, infection, etc.).
This test should be performed in the morning after at least three days of unrestricted diet (greater than 150 g of carbohydrate daily) and usual physical activity. A reasonable (30–50g) carbohydrate containing meal should be consumed on the evening before the test. The test should be preceded by an overnight fast of 10-16 hours, during which water may be drunk. Smoking is not permitted during the test.
Glucose load: |
75g of glucose – available from Pharmacy, dissolved in 300ml of water. |
PROTOCOL
| 1. | Take fasting blood sample for glucose (minimum 1ml into fluoride tube – grey top). |
| 2. | Give the appropriate glucose load (see previous page) orally over a period of 5 minutes. Care should be taken to avoid vomiting. Timing of the test is from the beginning of the drink. |
| 3. | Take further blood samples for glucose (minimum 1ml into fluoride tube – grey top) at 60, and 120 minutes after glucose ingestion. |
WHO DIAGNOSTIC VALUES
Pregnant women who meet WHO criteria for diabetes mellitus or IGT are classified as having Gestational Diabetes Mellitus (GDM).
WHO criteria - apply to venous, plasma only.
Gestational Diabetes mellitus: |
Fasting value |
>=7.0 mmol/L |
2hr after glucose load |
>=7.8 mmol/L |
Local management guidelines also classify patients as having gestational diabetes based on ADA criteria:
ADA criteria - apply to venous, plasma only.
Gestational Diabetes mellitus: |
Fasting value |
>=5.3 mmol/L |
1hr after glucose load |
>=10.0 mmol/L |
After the pregnancy ends, the woman should be re–classified as having either diabetes mellitus, or IGT, or normal glucose tolerance based on the results of a GTT six weeks or more after delivery. It should be emphasized that such women, regardless of the 6–week post–pregnancy result, are at increased risk of subsequently developing diabetes.
Reviewed by: Tony
Everitt, Consultant Biochemist
Last edited 07/08/08