Laboratory: Clinical Biochemistry  (AUTOMATED)
Test Name:
GLUCOSE CHALLENGE TEST (GCT), 50 g – GESTATIONAL DIABETES(GDM) - (P)
Test Code: GT50
Clinical Information:
Alternate Name(s): OGTT 50G-Pregnancy, Gestational Diabetes Screen
Description: The 50g oral glucose tolerance test is the 1st tier screening test for GDM, as defined in the Preferred Approach by Diabetes Canada Guidelines (2018).

 

The pregnant patient consumes a 50 g glucose drink in non-fasting state (any time of the day) and blood is collected at 1 hour post-drink for plasma glucose. The entire drink must be consumed within 5 minutes for the test results to be valid.

 

Time zero = once entire drink has been consumed

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


Test Indications: All pregnant patients 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.

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:  

·        The patient should not be dehydrated prior to testing.

·        The patient does not need to be fasting

·        The patient should be prepared to sit/rest for the entire duration of the test. No walking or exercise, as this may lead to falsely lower glucose results.

·        The 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.
 
Referral:
Serum: 1.0 mL
Requisition:
Reference Values:
Plasma glucose, mmol/LInterpretation
<7.8Normal.
>11.1Diagnosis of GDM. Confirmation with 75g OGTT is not required.
7.8-11.0Screen for GDM is positive, follow-up with confirmatory 75g OGTT.


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
Availability:
Weekdays, by Prior Arrangement Only
See Also:
More Information: