A 32-year-old multigravida at 24 weeks gestation reports a 1-hour glucose challenge test (GCT) result of 165 mg/dL (normal <140 mg/dL). She is asymptomatic with no prior history of gestational diabetes. What is the most appropriate next investigation?

Explanation

## Confirmatory Testing for Abnormal Glucose Challenge Test **Key Point:** A 1-hour GCT ≥140 mg/dL (or ≥135 mg/dL in some protocols) is a screening result that mandates a 3-hour OGTT for diagnosis of gestational diabetes mellitus (GDM). ### Diagnostic Algorithm for GDM Screening ```mermaid flowchart TD A[Pregnant woman 24-28 weeks]:::outcome --> B[1-hour GCT<br/>50 g glucose, fasting not required]:::action B --> C{GCT result}:::decision C -->|< 140 mg/dL| D[Low risk: no further testing]:::outcome C -->|≥ 140 mg/dL| E[Proceed to 3-hour OGTT<br/>100 g glucose, fasting required]:::action E --> F{OGTT values}:::decision F -->|≥ 2 abnormal values| G[Diagnosis: GDM]:::urgent F -->|0-1 abnormal values| H[Impaired glucose tolerance<br/>or normal]:::outcome ``` ### Why 3-Hour OGTT? **Sensitivity & Specificity:** - 1-hour GCT is a **screening test** (high sensitivity, lower specificity) - 3-hour OGTT is the **gold standard confirmatory test** for GDM - ~25% of women with abnormal GCT have normal OGTT **Diagnostic Criteria (100 g OGTT):** | Timepoint | Threshold (mg/dL) | | --- | --- | | Fasting | ≥95 | | 1 hour | ≥180 | | 2 hours | ≥155 | | 3 hours | ≥140 | **Diagnosis:** ≥2 abnormal values = GDM **High-Yield:** The 1-hour GCT is NOT diagnostic; it is a screening tool. An abnormal result always requires OGTT confirmation before GDM is diagnosed and treatment initiated. **Clinical Pearl:** Fasting is NOT required for the 1-hour GCT (screening), but IS required for the 3-hour OGTT (confirmatory). [cite:Park 26e Ch 6; ACOG Practice Bulletin 190 (Gestational Diabetes Mellitus)]