Every blood glucose reading tells you about a single moment—what's happening right now, this second, in your bloodstream. But your body has been quietly keeping a longer record, one that spans months rather than minutes.

Hemoglobin A1c, often called HbA1c or simply A1c, has become the cornerstone of diabetes care precisely because it captures what individual glucose checks cannot: the average of your blood sugar over roughly three months. This single number influences diagnosis, treatment decisions, and risk assessment for complications.

Yet the mechanism behind this test—and its surprising limitations—remain mysterious to most people who receive their results. Understanding how glucose literally attaches to your red blood cells, and what can throw off that measurement, transforms A1c from an abstract percentage into a meaningful window on your metabolic health.

Your red blood cells are essentially hemoglobin delivery vehicles, shuttling oxygen from lungs to tissues. But hemoglobin has an unintended property: glucose molecules in your blood gradually attach to it through a process called glycation.

This isn't an enzymatic process—it happens spontaneously, without any biological machinery directing it. Glucose simply drifts into contact with hemoglobin and forms a chemical bond. The higher your blood glucose, the more frequently these collisions occur, and the more glycated hemoglobin accumulates.

Red blood cells live approximately 120 days before the spleen filters them out and new ones replace them. This lifespan creates the three-month window: your current A1c reflects the glucose exposure of cells at various ages, from brand new to nearly retired. More recent weeks contribute more heavily—roughly half your A1c comes from the past 30 days.

The test measures what percentage of your hemoglobin has glucose attached. An A1c of 6% means six out of every hundred hemoglobin molecules have been glycated. This percentage correlates with average glucose: a 6% A1c corresponds to roughly 126 mg/dL average, while 7% suggests around 154 mg/dL.

Takeaway

Your red blood cells act as passive glucose recorders—the more sugar in your blood, the more permanently marks them. A1c reads that record.

A1c assumes your red blood cells have a normal lifespan and your hemoglobin behaves typically. When either assumption fails, the number becomes unreliable—sometimes dangerously so.

Conditions that falsely lower A1c generally involve shortened red blood cell survival. Hemolytic anemias, significant blood loss, and conditions causing rapid red cell turnover all mean your cells don't live long enough to accumulate expected glycation. Someone with chronic hemolysis might show a reassuring A1c while actually running high glucose.

Conditions that falsely raise A1c often involve the opposite: cells living longer than normal, or modified hemoglobin that glycates more readily. Iron deficiency anemia, certain hemoglobin variants common in specific ethnic populations, and kidney disease can all push A1c higher than true glucose exposure would suggest.

This matters clinically. Hemoglobin variants like HbS (sickle cell trait), HbC, and HbE affect millions of people worldwide and can interfere with certain A1c testing methods. If your results seem inconsistent with your glucose logs, or if you have known hemoglobin abnormalities, discuss alternative monitoring like fructosamine or continuous glucose monitoring with your provider.

Takeaway

A1c accuracy depends on normal red blood cell biology. Anemia, hemoglobin variants, and kidney disease can all make the number misleading in either direction.

The diagnostic thresholds for diabetes weren't arbitrary—they emerged from population studies linking A1c levels to the appearance of diabetic complications, particularly the distinctive retinal damage visible in eye exams.

Below 5.7% is considered normal. 5.7-6.4% indicates prediabetes, a metabolic state where insulin resistance has begun but full diabetes hasn't developed. 6.5% or higher meets the diagnostic threshold for type 2 diabetes when confirmed on repeat testing.

But these cutoffs simplify a continuous relationship. Risk doesn't suddenly appear at 6.5%—it rises gradually. Someone at 5.8% has meaningfully higher cardiovascular risk than someone at 5.2%, even though both fall into the "prediabetes" category. Each 1% increase in A1c above normal correlates with roughly 15-20% increased risk of cardiovascular events.

For people with diagnosed diabetes, treatment targets typically aim for below 7% to reduce complication risk, though individualized goals matter. Tighter control (6.5% or lower) offers additional protection for some, while older adults or those with multiple health conditions may appropriately target 7.5-8% to avoid dangerous low blood sugar episodes from intensive treatment.

Takeaway

A1c thresholds define diagnostic categories, but risk exists on a spectrum. Understanding where you fall—and the direction you're trending—matters more than any single cutoff.

Your A1c offers something no single glucose reading can: perspective. It reveals the metabolic environment your blood vessels, nerves, and organs have been swimming in for months.

But it's a tool with assumptions baked in. Knowing that anemia distorts results, that recent weeks count more than distant ones, and that diagnostic thresholds represent points on a continuous risk curve—this context transforms a percentage into actionable intelligence.

Track the trend over time. Question results that don't match your glucose logs. And remember that this three-month window exists to help you see patterns your daily measurements might miss.