Reassessment of the Diagnostic Accuracy of HbA 1c and Glucose for Type 2 Diabetes: A Systematic Review and Meta‐Analysis of Observational Studies

Aims This systematic review and meta‐analysis evaluated the diagnostic performance of HbA 1c , fasting plasma glucose (FPG), and 1‐h plasma glucose (1hPG) against the 2‐h plasma glucose standard (≥ 11.1 mmol/L) in adults without previously diagnosed diabetes. Methods A comprehensive search of Medline and CENTRAL (until 09.2025) identified population‐based studies considering all available cutoff values for these markers. Diagnostic accuracy was estimated using bivariate random‐effects models. Summary ROC curves and optimal cutoff values were derived using a Weibull accelerated failure time approach and the Youden index. Subgroup analyses explored potential differences across continents. The certainty of evidence (CoE) was evaluated using the GRADE approach. Results Fifty‐eight studies including 276,203 participants were synthesised. Overall, FPG showed higher diagnostic accuracy than HbA 1c , with an optimal cutoff of 6.3 mmol/L [95% confidence interval 6.0 mmol/L; 6.5 mmol/L], yielding 73% [68%; 78%] sensitivity and 91% [89%; 94%] specificity. The optimal HbA 1c cutoff was 47 [42; 49] mmol/mol (i.e., 6.5% [6.0%; 6.6%]), with 62% [51%; 80%] sensitivity and 92% [77%; 96%] specificity. The CoE was low. Across all regions, FPG consistently outperformed HbA 1c . Optimal FPG cutoff values ranged from 5.3 mmol/L in North America to 6.5 mmol/L in Asian countries, and from 41 mmol/mol (5.9%) in Asian countries to 45 mmol/mol (6.3%) in North America for HbA 1c . Conclusions These results suggest that FPG is a more reliable diagnostic marker than HbA 1c and that uniform global thresholds may not fully reflect population‐specific differences. Further high‐quality studies are needed to refine diagnostic performance and examine factors influencing accuracy.