Glucose Tolerance Test

Testing Laboratory

Auckland Te Toka Tumai | Auckland Hospital

Department

Chemical Pathology

Delphic Registration Code (Alpha Code)

GTTF (FASTING)

Test Code (Numeric Code)

0042

Laboratory Handling

Phlebotomy

See additional information. Note: Two bloods are to be collected a) Fasting blood (before glucose drink) b) 2 hours post-glucose drink.

Transport

Fluoride tubes should be used if specimens are expected to take >1 hour to arrive at the lab.

Registration

Labplus Order Code: Alpha Code:GTT2 (2HRS), Numeric Code:0041 If registration form has glucose tolerance test and serum glucose selected/written only register for glucose tolerance test.

Test Adds Requirement

PST/Heparin tubes are acceptable if Fluoride tube is not available. Please check sample collection time prior to test add.

Sample Stability

See glucose test for details

Synonyms

GTT

Turnaround Time

3 hours

Specimen Type

• Plasma

Specimen Requirements

Tube/Container Type:

Preferred

• Fluoride (Grey Cap)

Paediatric Specimen Requirements

Tube/Container Type:

Preferred

• Fluoride (Grey Cap)

Specimen Collection
The following criteria are from the Australian Diabetes Society, NZ Society for the Study of Diabetes, the Royal College of Pathologists of Australasia, and the Australasian Association of Clinical Biochemists (Ref: New classification and criteria for diagnosis of diabetes mellitus. PG Colman et al. MJA 1999; 170: 8)

Prior to commencement of the test, a random or fasting blood glucose must be tested.

The glucose tolerance test must NOT be performed if:

1. The fasting glucose is >9 or random glucose >13 mmol/L

2. The patient is very ill (the test is invalid in the presence of intercurrent illness or stress such as that induced by trauma, surgery etc.).

3. The patient is a known diabetic.

PROCEDURE for performing a Glucose tolerance test:
Note: Two bloods are to be collected
a) Fasting blood (before glucose drink)

b) 2 hours post-glucose drink

1. For three days prior to the test the patient must be on a diet containing more than 150 g of carbohydrate daily.

2. Drug therapy should be stopped on the morning of the test.

3. The patient must not smoke during the test.

4. Following a fast of at least 8h, an oral dose of glucose is given.

Dose:

Adult: 75 g anhydrous glucose in 300-350 mL water
Pregnant Women: 75 g anhydrous glucose in 300-350 mL water

Also see Gestational Diabetes for details of the non-fasting 50 g oral glucose screening test (Polycose test)

Children: 1.75 g/kg of body weight (maximum dose 75g)

5. Two blood specimens are collected: fasting, and two hours after glucose.

Contact Information