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Laboratory: Clinical Biochemistry  (AUTOMATED)
Test Name:
Test Code: GTT2
Clinical Information:
Alternate Name(s): Glucose Tolerance 2 hour

Description: Fasting plasma glucose, A1C, and/or 2-hour plasma glucose in a 75 g OGTT are used to diagnose diabetes.
75 g OGTT testing includes 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 is start of the drink consumption, and 2-hour collection must be ±10 minutes.
Glucose dose:
Adult (non-pregnant): 75 g glucose
Child (<18 y.o. and <42 kg): adjust dose based on 1.75g/kg to a max of 75 g.

Test Indications: While fasting plasma glucose (FPG) and/or A1C are the recommended screening tests for type 2 diabetes, a 75 g oral glucose tolerance test (OGTT) may be considered when the FPG is 6.1 to 6.9 mmol/L and/or A1C is 6.0% to 6.4%. 
Additionally, instances when A1C may be unreliable include: individuals with various hemoglobinopathies, hemolytic or iron deficiency anemias, iron deficiency without anemia, Graves' disease and severe hepatic and renal disease, although some evidence suggests that A1C may not be affected by these conditions in people without diabetes.

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.
Serum: 1.0 mL
Reference Values:
Sample Prediabetes Diabetes
Fasting PG 6.1-6.9 mmol/L ≥ 7.0 mmol/L
2 h PG 7.8-11.0 mmol/L ≥ 11.1 mmol/L

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: