What are the normal glucose levels for a post-partum Glucose Tolerance Test (GTT) in Canada?

Normal Values for Post-Partum Glucose Tolerance Test in Canada

Women with a history of gestational diabetes mellitus (GDM) should be tested for prediabetes or diabetes 4-12 weeks postpartum using a 75-g oral glucose tolerance test (OGTT) with the following non-pregnancy diagnostic criteria: fasting plasma glucose ≥7.0 mmol/L (126 mg/dL) or 2-hour plasma glucose ≥11.1 mmol/L (200 mg/dL) for diabetes diagnosis. 1

Post-Partum GTT Diagnostic Criteria

The post-partum OGTT uses standard non-pregnancy criteria to identify persistent glucose abnormalities:

Normal Values:

• Fasting plasma glucose: <6.1 mmol/L (<110 mg/dL)
• 2-hour plasma glucose: <7.8 mmol/L (<140 mg/dL)

Prediabetes Values:

• Impaired fasting glucose (IFG): 6.1-6.9 mmol/L (110-125 mg/dL)
• Impaired glucose tolerance (IGT): 2-hour plasma glucose 7.8-11.0 mmol/L (140-199 mg/dL)

Diabetes Values:

• Fasting plasma glucose: ≥7.0 mmol/L (≥126 mg/dL) OR
• 2-hour plasma glucose: ≥11.1 mmol/L (≥200 mg/dL)

Timing of Post-Partum Testing

The 75-g OGTT should be performed at 4-12 weeks postpartum 1. This timing is critical because:

• Earlier testing (2-4 days postpartum) may have good specificity but lower sensitivity for detecting glucose abnormalities 2
• HbA1c is not recommended at this visit because the concentration may still be influenced by pregnancy-related changes and peripartum blood loss 1

Importance of Using OGTT vs. Fasting Glucose Alone

The complete 75-g OGTT (not just fasting glucose) is strongly recommended because:

• Using fasting plasma glucose alone detects only 15.8% of women with any glucose abnormality postpartum 3
• The 2-hour post-glucose load value detects 97.5% of abnormalities 3
• Fasting plasma glucose alone fails to detect IGT or diabetes in 39% of cases 3

Follow-Up Testing After Initial Post-Partum GTT

For women with a history of GDM:

• If initial post-partum GTT is normal: lifelong screening for diabetes every 1-3 years 1
• Follow-up testing can use any recommended glycemic test (annual HbA1c, annual fasting plasma glucose, or triennial 75-g OGTT) 1

Clinical Implications

Women with GDM have a significantly increased lifetime risk of developing type 2 diabetes:

• 20% at 10 years
• 30% at 20 years
• 40% at 30 years
• 50% at 40 years
• 60% at 50 years 1

This underscores the importance of proper post-partum testing and long-term follow-up to identify diabetes early and implement preventive strategies.

Common Pitfalls to Avoid

1. Relying solely on fasting glucose: This misses many cases of glucose intolerance and diabetes 3
2. Using HbA1c for initial post-partum testing: Pregnancy-related changes affect HbA1c values 1
3. Poor follow-up rates: Only about 50% of women return for the recommended 6-12 week post-partum testing 2, 4
4. Ignoring women with only one abnormal GTT value during pregnancy: These women still have increased risk compared to those with completely normal glucose tolerance 5

The complete OGTT at 4-12 weeks postpartum remains the gold standard for detecting persistent glucose abnormalities after GDM.