The 75g OGTT is the second-tier test for gestational diabetes mellitus (GDM), as defined in the Preferred Approach by Diabetes Canada (2018 Guidelines).

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 1 hour and 2 hours post-drink for plasma glucose.

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

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

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

Test Indications: All pregnant women without known pre-existing diabetes should be screened for gestational diabetes mellitus (GDM) at 24 – 28 weeks of gestation. GDM is defined as hyperglycemia with onset or first recognition during pregnancy. The global prevalence of hyperglycemia during pregnancy has been estimated at 16.9% using WHO criteria.  Between 3% to 20% of pregnant women develop gestational diabetes.
 
Untreated GDM carries short-term (preeclampsia, preterm delivery) and long-term (obesity, diabetes, cardiovascular disease) health risks for both mother and baby, while treatment reduces these adverse outcomes.

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 

PG, plasma glucose

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