A 42-year-old man from urban Delhi presents for routine health screening. His fasting blood glucose is 118 mg/dL and random blood glucose is 156 mg/dL. He is asymptomatic with no prior diabetes diagnosis. Which is the most appropriate investigation of choice to confirm diabetes mellitus in this patient?

Explanation

## Diagnostic Approach to Suspected Diabetes ### Classification of Blood Glucose Values The patient's fasting glucose (118 mg/dL) falls in the **impaired fasting glucose (IFG)** range (100–125 mg/dL), which is neither normal nor diagnostic of diabetes on a single reading. Confirmation requires additional testing. ### Investigation of Choice: HbA1c **Key Point:** HbA1c is now the WHO-recommended first-line confirmatory test for diabetes diagnosis because it: - Reflects average blood glucose over 2–3 months - Does NOT require fasting - Has minimal day-to-day biological variation - Is unaffected by acute stress or illness (unlike single glucose readings) - Can be used for both diagnosis AND monitoring ### Diagnostic Criteria Using HbA1c | HbA1c Level | Interpretation | |---|---| | < 5.7% | Normal | | 5.7–6.4% | Prediabetes | | ≥ 6.5% | Diabetes mellitus | **High-Yield:** HbA1c ≥ 6.5% is diagnostic of diabetes in a symptomatic patient or when confirmed on repeat testing in asymptomatic individuals. ### Why HbA1c Over Other Tests? 1. **Standardization:** HbA1c has uniform diagnostic thresholds across all populations (unlike glucose, which varies by fasting status) 2. **Convenience:** No fasting required; can be done anytime 3. **Stability:** Less affected by acute illness, stress, or medications 4. **Reproducibility:** Single elevated HbA1c in asymptomatic patient requires confirmation; two elevated values = diagnostic **Clinical Pearl:** In India, HbA1c cutoff of ≥ 6.5% is recommended by RSSDI (Revised Indian Diabetes Society) for diabetes diagnosis, aligning with WHO/ADA guidelines. ### Alternative Tests (Less Preferred for Initial Diagnosis) - **Repeat fasting glucose:** Requires two separate fasting readings; more cumbersome - **OGTT:** Gold standard for gestational diabetes; less convenient for routine screening - **Fasting insulin:** Used for insulin resistance assessment, NOT diabetes diagnosis [cite:Park 26e Ch 5]