A blood test can tell a person with diabetes that their blood sugar has been under control for the past three months. The numbers look reassuring, the report carries no obvious warning, and the patient may believe that their treatment is working. But there is a catch: sometimes, the HbA1c reading itself can be misleading.This matters because HbA1c is one of the most commonly used tests to diagnose and monitor diabetes. Yet, despite its importance, it does not tell the complete story of a person’s blood sugar control.
What does HbA1c actually measure?
To understand why the report can sometimes be misleading, it helps to know what the test measures.When glucose circulates in the bloodstream, some of it attaches to haemoglobin, the protein inside red blood cells that carries oxygen. This process is called glycation. The HbA1c test measures the percentage of haemoglobin that has glucose attached to it.An HbA1c level of 6.5% or higher falls in the diabetes range, while 5.7% to 6.4% indicates prediabetes. A result below 5.7% is generally considered within the normal range.The problem begins when something affects the red blood cells themselves. If they survive for a shorter or longer period than usual, or if the haemoglobin structure is different, the amount of glucose attached to haemoglobin may not accurately represent the person’s actual blood sugar levels.Dr Jayanta Thakuria, Senior Director – Internal Medicine and Rheumatology, Yatharth Super Speciality Hospital, Faridabad Sector 88, explains:”Factors that alter the production or lifespan of red blood cells may make HbA1c reports unreliable. Some of the examples of such disease conditions include vitamin B12 deficiency anemia, iron deficiency anemia, recent blood loss, blood transfusion, renal diseases, and hemoglobinopathies. Because of the effect of these debilitating conditions on red blood cells, HbA1c may not provide reliable results, and thus, additional tests may help get proper evaluation.”
What HbA1c does not tell you about daily blood sugar levels
Even when the test is accurate, it has another limitation: it does not show how blood sugar behaves throughout the day.A person could have frequent spikes after meals and episodes of low blood sugar at night, yet their average glucose reading may appear relatively stable. This is because the test captures an average rather than the individual highs and lows.Dr Thakuria explains, “HbA1c is a measurement of blood glucose that shows average levels for about three months but does not report daily activities or processes. There are no findings of drastic rises after meals, instances of being low in blood sugar, or highs and lows during the night. In such cases, monitoring of blood glucose is essential.”This distinction matters particularly for people taking insulin or medicines that can cause hypoglycaemia (low blood sugar). An average reading alone may not reveal how frequently their glucose levels are dropping or rising.The Centers for Disease Control and Prevention (CDC) also points out that HbA1c cannot capture sudden, temporary changes in blood sugar. Readings from the past 30 days influence the result more heavily, but individual fluctuations can still remain hidden.The larger message is that diabetes management cannot always be reduced to one number on a laboratory report. HbA1c remains a valuable tool, but its accuracy depends partly on the health of the red blood cells carrying the haemoglobin.
