A 24-year-old primigravida at 24 weeks of gestation presents for routine antenatal care. She is asymptomatic with no significant past medical history. As part of screening for gestational diabetes mellitus, which is the most appropriate investigation of choice?

Question

Accepted Answer

C. 50-gram glucose challenge test (GCT). ## 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.

Answer

A. Glycated hemoglobin (HbA1c)

Answer

B. Oral glucose tolerance test (OGTT) with 75 grams glucose

Answer

D. Fasting blood glucose

A 24-year-old primigravida at 24 weeks of gestation presents for routine antenatal care. She is asymptomatic with no significant past medical history. As part of screening for gestational diabetes mellitus, which is the most appropriate investigation of choice?

Explanation

Screening for Gestational Diabetes

Rationale for 50-gram GCT

Diagnostic Algorithm

Why Other Tests Are Not First-Line

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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