During a population-based diabetes surveillance survey in an urban Indian cohort, 500 individuals with newly detected impaired fasting glucose are identified. The epidemiologist must assess the prevalence of undiagnosed diabetes in this subgroup. Which single investigation is most appropriate for large-scale population screening in this context?

## Investigation of Choice for Population-Level Diabetes Screening **Key Point:** For large-scale population screening and surveillance, HbA1c is the most practical and cost-effective investigation despite OGTT being the diagnostic gold standard in clinical settings. ### Why HbA1c Is Optimal for Population Screening **Advantages of HbA1c in epidemiological surveys:** 1. **No fasting required** — eliminates logistical burden in community settings. 2. **No glucose load needed** — removes the need for standardized 75 g glucose administration and 2-hour wait. 3. **Single blood draw** — rapid processing, minimal subject inconvenience. 4. **Standardized across labs** — NGSP (National Glycohemoglobin Standardization Program) certification ensures reproducibility. 5. **Reflects 3-month glycemic average** — less affected by acute stress, illness, or day-to-day variation. 6. **Cost-effective at scale** — lower per-subject cost for 500 individuals compared to OGTT logistics. ### Diagnostic Criteria by HbA1c (ADA & WHO 2011) | HbA1c Range | Interpretation | |---|---| | < 5.7% (< 39 mmol/mol) | Normal | | 5.7–6.4% (39–47 mmol/mol) | Prediabetes | | ≥ 6.5% (≥ 48 mmol/mol) | Diabetes | **High-Yield:** HbA1c ≥ 6.5% is now an accepted diagnostic criterion for diabetes by ADA, WHO, and Indian guidelines (ICMR-INDIAB 2015). It is particularly useful in population surveys where repeat testing is impractical. **Clinical Pearl:** In the ICMR-INDIAB study, HbA1c-based screening identified a higher prevalence of undiagnosed diabetes in urban Indian populations compared to FBG alone, making it superior for surveillance epidemiology. **Mnemonic: HbA1c-SCALE** — *HbA1c is Suitable for Community Assessment of Large-scale Epidemiology* ### Why OGTT Is Impractical Here Although OGTT is the gold standard in clinical diagnosis, it is **not feasible for population surveys** because: - Requires fasting state (reduces participation). - Mandates 75 g glucose load and 2-hour observation (time-intensive). - Higher cost per subject for 500 individuals. - Logistically complex in field settings.