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HbA1c is the most widely used marker for diagnosing and monitoring diabetes. It reflects average blood glucose over the previous three months by measuring the percentage of haemoglobin that has been glycated, essentially, how much sugar has attached itself to your red blood cells over their lifespan. It’s convenient, doesn’t require fasting, and gives a longer-term picture than a single blood glucose reading. But there’s a critical blind spot that most patients and frankly, many clinicians overlook. HbA1c assumes your red blood cells are behaving normally. When they aren’t, the number on your report can be dangerously misleading.
HbA1c depends on haemoglobin being exposed to glucose over the typical 120-day lifespan of a red blood cell. Any condition that alters the lifespan or changes haemoglobin levels distorts the result. “Iron deficiency anaemia, the most common form in India, slows red blood cell turnover. Older red blood cells spend more time in circulation, accumulating more glucose on their surface,” Dr Gagandeep Singh tells Health Shots. A falsely elevated HbA1c that suggests worse blood sugar control than actually exists. A woman with well-managed glucose levels may be told her diabetes is worsening, leading to unnecessary medication escalation.
Conversely, haemolytic anaemia, thalassaemia traits, and conditions that cause rapid red blood cell destruction shorten that lifespan. Less time in circulation means less glycation, producing a falsely low HbA1c. This is the more dangerous scenario: a patient with genuinely high blood sugar may receive a reassuringly normal report, delaying diagnosis entirely.
A recent Lancet paper has specifically flagged this concern for India, noting that reliance on HbA1c alone can misclassify diabetes in populations with high anaemia prevalence and haemoglobin disorders, precisely the demographic profile we see across the country.
According to NFHS-5 data, 57% of Indian women aged 15–49 are anaemic, and 25% of men fall into the same category. We are simultaneously one of the world’s largest populations for both anaemia and diabetes. This overlap means millions of Indians are receiving HbA1c readings that may not reflect their actual metabolic status.
“In my practice, I routinely encounter patients whose HbA1c and continuous glucose monitoring data don’t align. A patient shows an HbA1c of 6.2%, seemingly well-controlled, but their CGM reveals regular post-meal spikes above 200 mg/dL. Or the reverse an HbA1c of 7.5% that alarms everyone, but glucose readings are consistently within range, and the patient turns out to have undiagnosed iron deficiency,” says Dr Singh.
First, never rely on HbA1c in isolation. If you’re being monitored for diabetes or prediabetes, ensure your physician is also checking your complete blood count, ferritin levels, and red blood cell indices. Any discrepancy between your HbA1c and day-to-day glucose readings should prompt an investigation to determine whether anaemia is skewing the HbA1c result.
Second, if you are anaemic, correct the anaemia before making major treatment decisions based on HbA1c. Research in Acta Biomedica suggests that it can take up to 6 months after iron correction for HbA1c to accurately reflect true glycaemic status.
Third, and this is where most conventional monitoring stops short, consider markers that bypass haemoglobin entirely. Fasting insulin levels and HOMA-IR (a calculation based on fasting insulin and glucose) directly measure insulin resistance, independent of red blood cell behaviour. This matters because insulin resistance is the root metabolic dysfunction driving Type 2 diabetes, often present years before blood sugar rises enough to register on an HbA1c test.
Diabetes management, and more importantly, diabetes reversal, depend on accurate data. If an undiagnosed nutritional deficiency is distorting the foundational measurement we use to assess metabolic health, we are building treatment plans on unreliable ground. In a country where anaemia and diabetes coexist at epidemic proportions, this isn’t an academic concern.
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