A 24-year-old primigravida at 24 weeks of gestation presents for routine antenatal screening. She has no symptoms but her random blood glucose is 145 mg/dL. What is the most appropriate investigation of choice to diagnose gestational diabetes mellitus?

Explanation

## Screening and Diagnosis of Gestational Diabetes **Key Point:** The 75-gram OGTT is the gold standard and investigation of choice for diagnosis of gestational diabetes mellitus (GDM) in India and most international guidelines. ### Diagnostic Algorithm **High-Yield:** The two-step approach recommended by ACOG and adopted in India: 1. **Step 1 (Screening):** 50-gram glucose challenge test (non-fasting) at 24–28 weeks 2. **Step 2 (Confirmation):** If screening is abnormal (≥140 mg/dL), proceed to 75-gram OGTT ### 75-gram OGTT Diagnostic Criteria (WHO 2013 / India) | Timepoint | Threshold (mg/dL) | Threshold (mmol/L) | |-----------|-------------------|--------------------| | Fasting | ≥92 | ≥5.1 | | 1 hour | ≥180 | ≥10.0 | | 2 hours | ≥153 | ≥8.5 | **Clinical Pearl:** GDM is diagnosed if **any one value** exceeds the threshold on 75-gram OGTT. This patient's random glucose of 145 mg/dL warrants formal OGTT confirmation. ### Why 75-gram OGTT? 1. **Specificity:** Directly measures glucose tolerance under standardized conditions 2. **Reproducibility:** Standardized test with established cutoffs 3. **Guideline-backed:** Recommended by WHO, ACOG, FIGO, and Indian guidelines (ICMR, FOGSI) 4. **Prognostic value:** Predicts both maternal and fetal complications **Mnemonic:** **OGTT = Gold standard** — Oral glucose tolerance test is the definitive diagnostic test for GDM.