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Laboratory: Clinical Biochemistry  (AUTOMATED)
Test Name:
Test Code: GTTP
Clinical Information:
Alternate Name(s): Glucose tolerance test (gestational)

Description: This test is performed as a follow-up test for positive 50 g Glucose Challenge Test (GCT).
75g OGTT is the 2nd tier test for GDM, as defined in Preferred Approach by 2018 Diabetes Canada Guidelines
75 g OGTT includes 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 is start of the drink consumption, and timed collection must be ±10 minutes.

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.

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 
Collection Devices:
Specimen Required:

Plasma: 0.2 mL

Special Processing: Specimen must be centrifuged, and plasma separated from cells within 30 min after collection. Delay (>30 minutes) causes decrease in glucose levels and potentially missed diagnosis of diabetes. On average, plasma glucose decreases 5-7% per hour due to glycolysis.
Serum: 1.0 mL
Reference Values:
GDM is diagnosed using one or more of the following criteria:
Sample Criteria
Fasting PG ≥5.3 mmol/L
1 h PG ≥10.6 mmol/L
2 h PG ≥9.0 mmol/L
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
Weekdays, by Prior Arrangement Only
See Also:
More Information: