## Screening for Gestational Diabetes **Key Point:** The 50-gram glucose challenge test (GCT) is the standard screening test for gestational diabetes in India and most countries, performed at 24–28 weeks of gestation regardless of fasting status. ### Rationale for 50-gram GCT 1. **Non-fasting test** — Convenient, no dietary restriction required 2. **High sensitivity (~80%)** — Detects approximately 80% of GDM cases 3. **Universal screening** — Recommended for all pregnant women at 24–28 weeks 4. **Cost-effective** — Inexpensive and widely available 5. **Threshold for referral** — Blood glucose ≥140 mg/dL (7.8 mmol/L) requires confirmatory OGTT ### Diagnostic Algorithm ```mermaid flowchart TD A[Pregnant woman at 24-28 weeks]:::outcome --> B[50g GCT non-fasting]:::action B --> C{Glucose ≥140 mg/dL?}:::decision C -->|No| D[GDM screening negative]:::outcome C -->|Yes| E[Proceed to 75g or 100g OGTT]:::action E --> F{OGTT diagnostic criteria met?}:::decision F -->|Yes| G[Gestational Diabetes Diagnosed]:::urgent F -->|No| H[GDM excluded]:::outcome ``` **High-Yield:** About 15–25% of pregnant women will have GCT ≥140 mg/dL and require confirmatory testing; only 15–20% of these will have true GDM on OGTT. ### Why Other Tests Are Not First-Line | Investigation | Reason Not Used for Screening | |---|---| | **Fasting blood glucose** | Requires fasting; less sensitive; not standard screening protocol | | **75g OGTT** | Confirmatory test, not screening; more time-consuming; reserved for abnormal GCT | | **HbA1c** | Reflects average glucose over 2–3 months; insensitive for acute GDM; not recommended for diagnosis | **Clinical Pearl:** If GCT is ≥140 mg/dL, the next step is a 75-gram OGTT (or 100-gram OGTT in some centers) for diagnosis, not repeat GCT. **Warning:** Do not confuse screening (GCT) with diagnosis (OGTT). Many candidates incorrectly choose OGTT as the first investigation.
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