How can late-onset gestational diabetes be detected if initial screening (Glucose Challenge Test (GCT)) occurs between 24-28 weeks?

Detecting Late-Onset Gestational Diabetes After Initial Screening

Women who develop clinical signs or symptoms suggestive of gestational diabetes after a negative initial screening at 24-28 weeks should undergo a repeat OGTT, as this can identify an additional 23.5% of GDM cases that would otherwise be missed. 1

Risk Factors for Late-Onset GDM

Women at higher risk for developing late-onset GDM (after initial negative screening) include those with:

• History of previous GDM (2.6 times higher risk) 1
• Higher glucose values on initial screening (even if below diagnostic threshold) 1
• Early abnormal glucose metabolism (fasting glucose 110-125 mg/dL or A1C 5.9-6.4%) 2
• Marked obesity 3
• Strong family history of diabetes 3
• Previous macrosomic infant 3

Clinical Indicators for Repeat Testing

Repeat OGTT should be performed when any of these clinical signs develop after initial negative screening:

• Fetal growth abnormalities (most common indicator, yields 22.7% positive results) 1
• Polyhydramnios
• Glycosuria
• Symptoms of hyperglycemia (polydipsia, polyuria)
• Recurrent infections (especially urinary tract or vaginal)

Approach to Repeat Testing

1. Timing: Perform repeat testing as soon as clinical signs or symptoms develop in the third trimester 1

2. Method: Use the same diagnostic test as recommended for initial screening:

   • Two-step approach: 50g glucose challenge test (GCT) followed by 100g OGTT if abnormal
   • One-step approach: 75g OGTT directly 3

3. Diagnostic criteria: Same as initial screening

   • For 75g OGTT: Fasting ≥92 mg/dL, 1-hour ≥180 mg/dL, or 2-hour ≥153 mg/dL (one abnormal value diagnostic) 3
   • For 100g OGTT: Fasting ≥95 mg/dL, 1-hour ≥180 mg/dL, 2-hour ≥155 mg/dL, 3-hour ≥140 mg/dL (two abnormal values diagnostic) 3

Predictive Values from Initial Screening

Initial OGTT values can help identify women at higher risk for late-onset GDM:

• Fasting glucose ≥4.8 mmol/L on initial OGTT: PPV 32%, NPV 83% 1
• 2-hour glucose ≥6.5 mmol/L on initial OGTT: PPV 47%, NPV 90% 1

Clinical Implications

Early detection and treatment of late-onset GDM is crucial as it reduces risks of:

• Preeclampsia
• Macrosomia
• Shoulder dystocia
• Other adverse pregnancy outcomes 3

Pitfalls to Avoid

• Assuming a negative initial screening eliminates all risk of GDM
• Ignoring clinical signs of GDM that develop after initial screening
• Failing to consider repeat testing in women with borderline results on initial screening
• Missing opportunities for intervention in women who develop late-onset GDM

The evidence clearly demonstrates that a single screening at 24-28 weeks may miss cases of GDM that develop later in pregnancy. Vigilance for clinical signs and symptoms, particularly in high-risk women, with prompt repeat testing can identify these cases and allow for timely intervention to improve maternal and fetal outcomes.