For a pregnant patient at 24–28 weeks, what is the protocol for gestational diabetes screening using only a fasting plasma glucose measurement?

Fasting-Only Protocol for Gestational Diabetes Screening at 24–28 Weeks

Using fasting plasma glucose alone at 24–28 weeks is not recommended and will miss the majority of gestational diabetes cases; you must perform the full oral glucose tolerance test with post-load measurements. 1

Why Fasting Glucose Alone Fails

• Fasting glucose alone misses 50–92% of GDM cases depending on the threshold used, because many women with gestational diabetes have normal fasting values but elevated post-prandial glucose levels that independently predict macrosomia, cesarean delivery, and neonatal complications. 2, 3

• A 2022 study demonstrated that using fasting glucose ≥92 mg/dL at 24–28 weeks missed 78% of GDM cases, while a threshold of ≥100 mg/dL missed 92.4% of cases. 2

• The 1-hour and 2-hour post-load values on the OGTT capture different pathophysiology than fasting glucose and are essential for identifying the full spectrum of glucose intolerance in pregnancy. 3

The Correct Protocol: Full OGTT Required

If you are screening at 24–28 weeks, you must use one of these complete protocols:

One-Step Approach (IADPSG/ADA—Preferred)

• Administer a 75-g glucose load after an 8–14 hour overnight fast. 3, 4

• Measure plasma glucose at three time points: fasting, 1 hour, and 2 hours. 3, 4

• Diagnostic thresholds: GDM is diagnosed when any single value meets or exceeds:

  • Fasting ≥92 mg/dL (5.1 mmol/L)
  • 1-hour ≥180 mg/dL (10.0 mmol/L)
  • 2-hour ≥153 mg/dL (8.5 mmol/L) 3, 4

• This approach identifies 15–20% of pregnancies with GDM and is based on the HAPO study showing continuous risk of adverse outcomes across the glucose spectrum. 3, 5

Two-Step Approach (ACOG-Supported Alternative)

• Step 1: 50-g non-fasting glucose challenge; if 1-hour glucose ≥130–140 mg/dL, proceed to Step 2. 3, 5

• Step 2: 100-g OGTT after overnight fast with measurements at fasting, 1,2, and 3 hours. 3, 5

• Diagnostic thresholds (Carpenter-Coustan): GDM diagnosed when at least two values meet or exceed:

  • Fasting ≥95 mg/dL (5.3 mmol/L)
  • 1-hour ≥180 mg/dL (10.0 mmol/L)
  • 2-hour ≥155 mg/dL (8.6 mmol/L)
  • 3-hour ≥140 mg/dL (7.8 mmol/L) 3, 5

• ACOG notes that in practice, a single elevated value may be used for diagnosis. 5, 4

• This approach identifies 5–6% of pregnancies with GDM but misses milder cases. 3, 5

Test Preparation Requirements

• Overnight fast of 8–14 hours is mandatory. 3, 5

• Patient must consume ≥150 g carbohydrate daily for 3 days before the test to ensure adequate glycogen stores. 3, 5

• Patient remains seated throughout the test and does not smoke. 3, 5

• Unrestricted physical activity in the days preceding the test. 3

Critical Pitfall to Avoid

Do not attempt to diagnose GDM at 24–28 weeks using only fasting glucose—this is not a validated approach and will result in massive underdiagnosis. The IADPSG guidelines explicitly require the full OGTT because the post-load values are essential for capturing the insulin resistance and impaired glucose tolerance characteristic of GDM. 1, 3

The only scenario where fasting glucose alone has diagnostic utility is early in pregnancy (first prenatal visit) when fasting glucose ≥92 mg/dL but <126 mg/dL can diagnose GDM, or ≥126 mg/dL indicates overt diabetes. 1 However, at 24–28 weeks—the standard screening window—you must perform the complete OGTT with all time points. 1, 3