Glucose tolerance test (GTT)

Chemical Pathology

Notes

• To diagnose diabetes mellitus, gestational diabetes or impaired glucose tolerance.
• If there are other clinical reasons for investigating glucose loading e.g. reactive hypoglycaemia or excessive growth hormone secretion then please contact the duty biochemist for alternative tests and protocols.

Precautions & Contraindications for the Test:

None.

Patient Preparation

• The test can be performed at the patient's surgery.
• It is important that the patient should have had normal meals during the last 3 days prior to the test and should not have been dieting.
• The test should be performed in the morning after an overnight fast [8 - 14 hours].
• The patient should be sitting quietly and must not smoke during the test procedure.
• False results may occur if the patient has recently been ill or has had prolonged bed rest.
• Water drinking is permitted during the period of fasting and during the test procedure.

Items Required

• 2x blood collection bottles: -

For adults and children: 2 x 2mL fluoride-oxalate tubes

For very young children: 2 x 0.25mL minicollect fluoride-oxalate tubes

• 1x bottle of POLYCAL [manufactured by Nutricia Clinical Care] - 113 mL of Polycal is equivalent to 75g of anhydrous glucose.
• Prepare the drink for the patient in advance as described below and keep in the fridge.

For ADULTS:

Dilute the Polycal as follows:

Make 113 mL of Polycal up to 200 mL with drinking water and mix.

For CHILDREN:

• Weigh the patient (children over 43 kg will require the full adult dose)
• 2.6 mL of Polycal is required for each kilogram body weight up to a maximum volume of 113 mL of Polycal

For example: a 30 kg child would require 30 x 2.6 = 78 mL of Polycal

• Dilute the volume of Polycal required for body weight with an equal volume of water and mix.

Test Procedure

1. Take blood into a grey top fluoride-oxalate tube and label with specimen time and as "FASTING" as well as the usual patient identification requirements.
2. Give the patient the prepared Polycal drink.
3. Note the clock time. The drink should be consumed within 5 minutes
4. Ask the patient to drink another 100 mL of water after finishing the Polycal drink.
5. Collect a further blood sample 2 hours after the Polycal drink was started. Label tube with full patient identification detail, "2 hour sample" and actual time of sample.
6. Full patient identification on sample tubes and request card should include surname, forename, date of birth and NHS or hospital number as well as the sample date and time.
7. Send all samples together with the request form to the laboratory.

Interpretation of Results

Useful Links

• [NG28] Type 2 diabetes in Adults: Management
• [NG17] Type I Diabetes in Adults: Diagnosis and Management
• WHO publications on diabetes
• [NG3] Diabetes in Pregnancy: management from preconception to the postnatal period
• GTT Patient information leaflet

Follow-up Tests If Diabetes Has Been Diagnosed

• Haemoglobin A1c
• Microalbumin
• Patients diagnosed with gestational diabetes should be offered lifestyle advice (including weight control, diet and exercise) and offered a fasting plasma glucose measurement (but not an oral glucose tolerance test) at the 6-week postnatal check and annually thereafter (please refer to guidelines above for full details).

Page last updated 24/04/2019