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The Oral Glucose Tolerance Test
W.H.O. Protocol
Indications for an Oral Glucose Tolerance Test (OGTT)
An OGTT is performed when the patient’s fasting glucose is equivocal (6.1 – 6.9 mmol/L), or during pregnancy to test for gestational diabetes.
An OGTT is not required if a patient’s fasting glucose is unequivocally in the diabetic range.
Patient Preparation
The OGTT should be administered in the morning after at least three days of unrestricted diet (greater than 150g of carbohydrate daily) and usual physical activity. The test should be preceded by an overnight fast of at least 12 hours, during which time only water may be drunk. Smoking is not permitted during the test. The presence of factors that may influence the interpretation of the results of the test must be recorded on the request form (e.g. medication, inactivity, infection, etc).
Test Protocol
- Confirm that the patient has fasted for at least 12 hours (water only) and write this on the request form.
- Using the Freestyle Precision Pro glucometer, perform a capillary blood glucose test according to the SOP/working instructions for the device. Record the result in the GTT diary and on the request form.
- If the glucometer glucose result is less than 3.0 mmol/L or more than 10.0 mmol/L: take a fasting venous blood glucose sample (fluoride oxalate tube) and send to the laboratory for analysis. The patient can then be sent home and advised to contact their GP to discuss the result. Do not proceed to do a glucose tolerance test (steps 4 - 9 below).
OR
- If the glucometer glucose result is 3.0 mmol/L to 10.0 mmol/L inclusive: proceed with the glucose tolerance test. Take a baseline glucose sample into a fluoride oxalate tube. Label the tube clearly with the date and (actual) time.
- Adult patients should drink 75g of anhydrous glucose in 250 - 300 mL of water over the course of 5 minutes.
- For children, the test load should be 1.75g of glucose per kg body weight up to a total of 75g of glucose.
- Patient may drink water freely, BUT MUST NOT EAT UNTIL THE TEST IS COMPLETE!
- Take a further sample for glucose into a fluoride oxalate tube 2 hours from the start of glucose consumption. Label the tube clearly with the date and (actual) time.
- Send both samples and the request form together in one bag.
NB:
The patient should not leave the hospital until the test is complete.
The test is invalidated if vomiting occurs.
If there is any doubt about whether or not to proceed to do the full glucose tolerance test, please seek the advice of the duty biochemist (ext. 153444).
Recommended Glucose Drinks
75 g anhydrous glucose in 250 – 300 mL cold water. Check carefully that the glucose is anhydrous - glucose obtained from pharmacies is often glucose.H2O, in which case 82.5g must be used.
Rapilose® OGTT oral solution (300 mL)
113 mL Polycal® liquid, diluted with extra water to give a total volume of 200-300mL. Polycal® liquid is flavoured and may be more palatable.
When calculating the Polycal® dose for children, Polycal® contains 0.66g anhydrous glucose per mL (or 1.51mL contains 1g anhydrous glucose).)
We do not recommend the use of Lucozade for OGTTs. This is due to variation in glucose concentration between different types and bottle size of Lucozade and frequent changes in the formulation by the manufacturer. Furthermore, Lucozade contains caffeine which may affect glucose metabolism.
Interpretation of OGTTs
Non-pregnant individuals
Normal:Fasting glucose ≤6.0 and 2-hour glucose <7.8 mmol/L
Diabetes:Fasting glucose 7.0 or 2-hour glucose 11.1 mmol/L
(Must have 2 abnormal glucose results on 2 separate days to confirm diagnosis in asymptomatic individuals).
Impaired fasting glucose (IFG):Fasting 6.1 – 6.9 and 2-hour glucose < 7.8 mmol/L
Impaired Glucose Tolerance (IGT)::Fasting < 7.0 and 2- hour glucose 7.8 – 11.0 mmol/L
In Pregnancy
In pregnancy, gestational diabetes is confirmed if the fasting glucose is 5.6 and/or the 2 hour glucose is 7.8 mmol/L.
NB: The above glucose values apply to venous plasma only. Different criteria apply for capillary whole blood.