Procedure:

The test involves the collection of fasting plasma glucose after an 8-hour fast, followed by consumption of glucose drink, and blood collected at 2 hours post-drink for plasma glucose.

Time zero = once entire drink has been consumed (must be consumed in 5 minutes)

2-hour collection must be ±10 minutes from expected collection time

Glucose dose:

Adult (non-pregnant): 75 g glucose

Child (<18 y.o. and <42 kg): adjust dose based on 1.75g/kg to a maximum of 75 g

Test Indications: This test is indicated in non-pregnant persons only.

Patients who have recently given birth should not have an OGTT performed until at least 6 weeks – 6 months postpartum, as per Diabetes Canada recommendations.

IMPORTANT NOTE: Patients who actively breastfeed during the test may exhibit falsely lower plasma glucose levels (~5% lower).

References:

J Obstet Gynaecol Can. Nov 2018. 40(11): 1484-1489.

Obstetrics & Gynecology. July 2012. 120(1): 136-143.

Recommendations: Diabetes Canada Clinical Practice Guidelines Expert Committee. Diabetes Canada 2018 Clinical Practice Guidelines for the Prevention and Management of Diabetes in Canada. Can J Diabetes. 2018;42(Suppl 1):S1-S325.
https://guidelines.diabetes.ca/cpg

Patient preparation instructions: Minimum 8h fasting (only water permitted)
Patient should not be dehydrated prior to testing
Patient should be prepared to sit/rest for the duration of the test. No walking or exercise.
Patient should bring a snack to have after test completion.
Some labs perform this test by appointments only. Call in advance to inquire.
Consult your physician if any of the medication should be discontinued prior to testing.
 
PB110-10-17 Patient Brochure Fasting GTT 

Critical Value: Glucose:
<2.5 mmol/L or >25 mmol/L
Method of Analysis: Glucose:
Roche Cobas - Photometric/Hexokinase + G6PDH
Ortho Vitros - Photometric/Glucose Oxidase + peroxidase