Analysis of 43,088 British Bangladeshi and Pakistani participants revealed that rs33950507, a variant causing HbE thalassemia trait, increases HbA1c levels by 0.13% without affecting actual blood glucose. This genetic variant, present in 5.8% of participants, led to 38% higher prediabetes risk and 11% higher type 2 diabetes diagnosis rates. Paradoxically, carriers showed 26% lower diabetic eye disease risk and 56% lower cerebrovascular disease risk, suggesting overdiagnosis and unnecessary treatment. Three-quarters of genetic carriers remained undiagnosed for thalassemia in health records. This finding exposes a critical blind spot in diabetes care affecting 350 million people globally with thalassemia traits. Current HbA1c-based diagnostic thresholds may systematically misclassify millions, particularly in populations where hemoglobin variants are common but underdiagnosed. The research suggests urgent need for precision medicine approaches and alternative diagnostic markers beyond HbA1c. However, as this preprint awaits peer review, these significant findings require validation before clinical practice changes. This represents potentially paradigm-shifting evidence for personalized diabetes diagnosis based on genetic background.