Oral glucose tolerance test for insulin resistance

Investigation Protocol

Indications

Diagnosis of diabetes, impaired glucose tolerance and insulin resistance

Preparation

Glucose solution is best prepared in advance as glucose dissolves better in warm water but is more palatable when cold. On the day prior to the test, dissolve 75 grams of anhydrous glucose (82.5 grams of glucose monohydrate, obtainable pre-weighed from pharmacy) in warm water and store in a fridge overnight.

The patient must be fasted from midnight (sips of water only).

Procedure

Check that the patient has fasted from midnight.

Perform venepuncture taking 5mls of blood and test a small sample of it using a glucose meter.

Place 2mls of the blood sample into a fluoride oxalate bottle, and 3mls into a lithium heparin tube, and label both tubes. Centrifuge the lithium tube immediately, separate and freeze the plasma, and take immediately to the laboratory.

If the result on the glucose meter is greater than or equal to 11mmol/l, send blood urgently to lab. If this is confirmed by biochemistry to be above 11mmol/l, stop the test, and allow the patient to go home.

If the result is less than 11mmol/l on meter, give the patient the glucose solution to drink (within 10 minutes). Collect a further blood sample in fluoride oxalate bottle for glucose and lithium heparin bottle for insulin measurements two hours after the glucose solution has been given. Centrifuge the lithium tube, separate and freeze the plasma, label as 120 minute sample, and take immediately to the laboratory.

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Patient information sheet

 Oral glucose tolerance test for insulin resistance

Your doctor has referred you to have an oral glucose tolerance test, which is performed over a two hour period. This is to establish whether or not you are developing diabetes, or whether your body has difficulty in using glucose (sugar) properly. 

We will take a sample of blood when you arrive in the unit. This will be tested for glucose concentration level and, if this is high, it will be unnecessary for the test to continue. Provided that the glucose level is not too high you will be given a drink of chilled glucose solution to be drunk over ten minutes. Following this a further sample of blood will be taken after two hours. You will be resting in a reclining chair in the unit throughout the test.

For three days prior to this test you should consume a normal diet. If you have been put on a weight reducing or low carbohydrate diet this must be stopped for three days. On the day of the test, it is important that you do not have anything to eat or drink from midnight, except sips of water. You will be offered a drink and something to eat at the end of the test.

You must not smoke on the day of the test. If you are taking any medicines or pills please check with our department to see if you should take them on the day of the test.

You should expect to stay at the hospital for about two and half hours. You may wish to bring a book or magazine to read. We do have a radio, or you can access Patientline; alternatively you may like to bring your own personal radio/cd player (please bring your own headphones). You may bring someone to stay with you during the test but there is not enough space for more than one person.

Children are discouraged, as this can be disruptive to other patients. However, if this poses a particular problem for you please discuss this with the senior nurse in the unit. You should be able to undertake normal activities after the test is finished and it should not interfere with your ability to drive home. 

If you have queries about the test, please contact:

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