Glucose Tolerance Test (GTT), (Extended, Non-Pregnant)

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Waitematā

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Glucose Tolerance Test (GTT), (Extended, Non-Pregnant)

Testing Laboratories

Waitematā | Northshore Hospital
Waitematā | Waitakere Hospital

Department

Chemical Pathology

Delphic Registration Code (Alpha Code)

GTT0, GTT1, GTT2 or GTT3

Test Code (Numeric Code)

0052

Laboratory Handling

Phlebotomy

Heparin samples prefered. Use flouride tube if > 2hr delay likely (e.g. Mason Clinic).

Registration

Use different lab number for each timed specimen. Fasting specimen must have lower lab number. Copy the request form for biochemistry. Test codes: Fasting (pre) - GTT0 1 hr post - GTT1 2 hr post - GTT2 3 hr post - GTT3"

Sample Stability

4 hours

Associated Tests

Glucose (Blood)

Synonyms

Glucose tolerance test

Turnaround Time

2 hours

Specimen Type

Plasma
Serum

Specimen Requirements

Tube/Container Type:

Preferred

Lithium Heparin - Non-gel (Green Cap)

Acceptable

Fluoride (Grey Cap)
Plain (Red Top)
SST II (Gold Cap)

Preferred Sample Volume (ml)

Minimum Sample Volume (ml)

No of Samples Required

Paediatric Specimen Requirements

Tube/Container Type:

Preferred

Heparin (Microtainer)

Acceptable

Plain(Microtainer)
SST(Microtainer)

Minimum Sample Volume (ml)

0.4

Pre-testing Requirements

Patient preparation:
Prior to commencement of the test, a random or fasting blood glucose must be sent to the Laboratory for analysis. If abnormally elevated, a GTT should NOT be performed.
Record fasting Glucocard result on request form.
Collect fasting glucose.

Procedure:
1. The patient should be well for 2 weeks prior and ideally have been on a diet containing more than 150 g of carbohydrate daily for the last three days.
2. The patient should not smoke or exercise within an hour prior to the test.
3. The patient preferably fasted for around 12 hours (definitely not 16 hours).

An oral dose of glucose is given:
Adults:Drink the entire contents of the 75g green 300ml Carbotest solution.

Children: Refer to Chemical pathology medical staff if GTT is requested for a child as the loading dose is lower.
(1.75/kg required = 7 mls Carbotest/kg up to a maximum of 300 ml Carbotest.)

Collect another fluoride tube for glucose estimation after 2 hours.

Specimens must be transported immediately to Biochemistry.

Patient must not eat, smoke excercise or drink for the duration of the test.

Diagnostic Use

Glucose tolerance test (GTT) ( 2 hours 75 grams glucose load)

The test is for determination of glucose tolerance status in non-pregnant adults or for confirmation of gestational diabetes in pregnant women.

NON- GESTATIONAL

Fasting <6.1 mmol/L and 2 hr <7.8 mmol/L	The GTT indicates normal glucose tolerance.
Fasting <6.1 mmol/L and 2 hr 7.8 – 11.0 mmol/L	The GTT indicates impaired glucose tolerance.
Fasting <6.1 mmol/L and 2 hr >11.0 mmol/L	The 2 hr level fulfils the criteria for diagnosis of diabetes. Note: if the 2 hr level is borderline (<12.0mmol/L) it is unwise to make a firm diagnosis on a single borderline GTT. Suggest repeat. Measurement of HbA1c may be informative.
Fasting ≥6.1 – <6.9 mmol/L and 2 hr <7.8 mmol/L	The results indicate impaired fasting glycaemia.
Fasting ≥6.1 – <6.9 mmol/L and 2 hr ≥7.8 – <11.0 mmol/L	The results indicate both impaired fasting glycaemia and impaired glucose tolerance. Measurement of HbA1c may be informative.
Fasting 6.1 – 6.9 mmol/L and 2 hr >11.0 mmol/L	The 2 hr level fulfils the criteria for diagnosis of diabetes.
Fasting ≥ 7.0 mmol/L and 2 hr <7.8 mmol/L	Fasting glucose fulfils the criteria for diagnosis of diabetes but it is unwise to make a firm diagnosis based on a single fasting glucose unless the level is unequivocally raised and/or there are symptoms of hyperglycaemia. A repeat fasting glucose is suggested. Measurement of HbA1c may be informative.
Fasting ≥ 7.0 and 2 hr ≥7.8 – <11.0 mmol/L	The 2 hour level indicates impaired glucose tolerance but the fasting glucose fulfils the criteria for diabetes. It is unwise to make a firm diagnosis on diabetes based on a single fasting glucose unless the level is unequivocally raised and/or there are symptoms of hyperglycaemia. Measurement of HbA1c may be informative.
Fasting ≥7.0 mmol/L and 2 hr ≥11.0 mmol/L	Both levels fulfil the criteria for diagnosis of diabetes.

2. GESTATIONAL

GTT at 24 – 28-weeks (earlier if deemed necessary based on the severity and number of risk factors)

Risk factors where direct 75 g GTT recommended include:

BMI >30 kg/m2 ; HbA1c 41 – 49 mmol/mol ; Age >30 ; PCOS ; Chronic hypertension ; Steroid or antipsychotic medication ; Family history of diabetes in a first degree relative ; LGA ; Non-white ethnicity
Previous: GDM; Macrosomia; Perinatal loss; Preterm birth

During pregnancy a person may develop risk factors that raise concern for GDM, direct 75g GTT and HbA1c recommended.

Interpretation:

Fasting level of ≥5.5 mmol/L OR a 2 hour level ≥9.0 mmol/L is consistent with the diagnosis of Gestational diabetes.
A borderline GTT is fasting at 5.1-5.4mmol/L and 2-hours at 8.5-8.9 mmol/L. If a borderline result is obtained, give basic lifestyle modification advice and a repeat GTT should be considered by 32 weeks especially when associated with high risk factors.
If both fasting and 2 hour results are <5.5 mmol/L AND <9 mmol/L respectively then this is a normal pregnancy GTT.

Last updated: 12/06/2026

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Laboratory Handling

Phlebotomy

Registration

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Associated Tests

Synonyms

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Specimen Requirements

Tube/Container Type:

Preferred

Acceptable

Preferred Sample Volume (ml)

Minimum Sample Volume (ml)

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Paediatric Specimen Requirements

Tube/Container Type:

Preferred

Acceptable

Minimum Sample Volume (ml)

Pre-testing Requirements

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Glucose Tolerance Test (GTT), (Extended, Non-Pregnant)

Overview

Testing Laboratories

Department

Delphic Registration Code (Alpha Code)

Test Code (Numeric Code)

Laboratory Handling

Phlebotomy

Registration

Sample Stability

Associated Tests

Synonyms

Turnaround Time

Specimen

Specimen Type

Specimen Requirements

Tube/Container Type:

Preferred

Acceptable

Preferred Sample Volume (ml)

Minimum Sample Volume (ml)

No of Samples Required

Paediatric Specimen Requirements

Tube/Container Type:

Preferred

Acceptable

Minimum Sample Volume (ml)

Pre-testing Requirements

Referral

Clinical Information/Test Method

Diagnostic Use

Additional Information

Resources

Contact Information

Contact Information

Enquiry Form

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