For decades, the HbA1c test has been treated as the gold standard for diagnosing and monitoring diabetes. A single blood draw, a single number, and doctors can estimate average blood sugar levels over three months. Simple. Efficient. Reliable.

But what if that number is quietly misleading millions?

A new evidence-based Viewpoint published in The Lancet Regional Health – Southeast Asia is challenging the near-universal reliance on HbA1c in South Asia. The analysis, led by Professor Anoop Misra of Fortis C-DOC Center of Excellence for Diabetes in New Delhi, warns that the test may not accurately reflect blood glucose levels in populations where anemia, inherited blood disorders, and G6PD deficiency are common — which includes vast sections of India.

HbA1c works by measuring glycated hemoglobin, essentially tracking how much sugar attaches to hemoglobin inside red blood cells. Because red blood cells typically live for about three months, the test provides an estimate of long-term glucose control. The science is sound — but only when hemoglobin itself is normal.

And that’s where the problem begins.

In India, iron deficiency anemia remains widespread, affecting large segments of the population. In some regions, more than half of people are nutritionally challenged. Add to that a significant burden of hemoglobinopathies and G6PD deficiency — inherited conditions that alter the structure or lifespan of red blood cells — and the reliability of HbA1c becomes far less certain.

“When conditions affect the quantity, structure, or lifespan of hemoglobin, HbA1c values can be distorted,” explains Professor Misra. “Relying exclusively on HbA1c can result in misclassification of diabetes status. Some individuals may be diagnosed later than they should be, while others could be misdiagnosed. That has serious implications for treatment and long-term outcomes.”

In practical terms, a patient’s blood sugar could be high, but their HbA1c may appear deceptively normal. Or the opposite may occur. In men with undetected G6PD deficiency, the authors note, dependence on HbA1c alone could delay a diabetes diagnosis by as much as four years — years during which silent damage to the heart, kidneys, nerves, and eyes may continue unchecked.

Dr. Shashank Joshi from Mumbai cautions that this is not merely a rural healthcare issue. “Even in well-resourced urban hospitals, HbA1c readings can be influenced by red blood cell variations and inherited hemoglobin disorders,” he says. “In rural and tribal populations, where anemia and red cell abnormalities are more common, discrepancies may be even greater.”

Compounding the issue is variability in laboratory quality control across settings, which can further affect test accuracy. The consequences extend beyond individual patients. If large-scale public health surveys rely solely on HbA1c, India’s true diabetes burden could be under- or overestimated, skewing policy decisions and resource allocation.

The authors are not calling for HbA1c to be abandoned. Instead, they are urging a shift from convenience-driven medicine to context-aware medicine.

In resource-limited settings, they suggest greater reliance on the oral glucose tolerance test, which measures blood sugar while fasting and again two hours after consuming 75 grams of glucose. For ongoing monitoring, self-monitoring of blood glucose using home glucometers several times a week, combined with basic blood tests to check hemoglobin levels, may provide a clearer picture.

In tertiary care centers, HbA1c should ideally be used alongside glucose tolerance testing, with continuous glucose monitoring where feasible. Alternative markers such as fructosamine may also offer additional insight. When clinical suspicion remains high, more detailed investigations — including iron studies, hemoglobin electrophoresis, and quantitative G6PD testing — can help uncover hidden distortions.

Dr. Shambho Samrat Samajdar from Kolkata underscores the need for a broader lens. “Combining glucose-based tests with hematologic assessments gives a more accurate picture of diabetes risk. This approach not only improves individual patient care but also strengthens public health data,” he says.

India is often described as the diabetes capital of the world. But as this analysis suggests, measuring that epidemic accurately may be more complicated than once believed. In a country where anemia and inherited blood disorders are common, a test built around hemoglobin cannot always be taken at face value.

The message from the authors is clear and urgent: one number cannot fit all. As diabetes rates climb, diagnostic strategies must adapt to the biological realities of the population. Because in the fight against a chronic disease that thrives on delay, even a few years of miscalculation can carry a heavy cost.