ORAL GLUCOSE TOLERANCE TEST
INTRODUCTION
Following a standard dose of glucose, plasma glucose is measured at 2 hours only and urines are not required. The results may be influenced by a number of factors and it is important that patient preparation instructions are carefully followed.
INDICATIONS
A fasting or random blood glucose sample is recommended as the initial investigation in suspected diabetes mellitus. The GTT should be used to confirm a diagnosis of diabetes mellitus if initial investigations prove equivocal.
PRECAUTIONS
Please refer to the Trust’s phlebotomy procedure for sample collection precautions.
The test should not be performed on seriously ill patients and those showing metabolic response to trauma and surgery. Reactive hypoglycaemia is a possibility and patients should be carefully supervised throughout the test.
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, oral contraceptives, 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. Recent evidence suggests that 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. |
Children: 1.75g of glucose per kg body weight up to a maximum of 75g. |
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 a further blood sample for glucose (minimum 1ml into fluoride tube – grey top) at 120 minutes after glucose ingestion. |
WHO DIAGNOSTIC VALUES
All values apply to venous, plasma only.
Diabetes mellitus: |
Fasting value |
>=7.0 mmol/l |
2hr after glucose load |
>=11.1 mmol/l |
|
Impaired glucose tolerance: |
Fasting value |
<7.0 mmol/l |
2hr after glucose load |
7.8 – 11.1 mmol/l |
Reviewed by: Tony
Everitt, Consultant Biochemist
Last edited 07/08/08