HbA1c Test · Blood

Your body runs on sugar, and every red blood cell keeps a chemical record of how much sugar has been floating through your bloodstream. HbA1c (hemoglobin A1c) is the percentage of your hemoglobin, the oxygen-carrying protein in red blood cells, that has permanently bonded with glucose. Because red blood cells live for about 120 days, this single number captures your average blood sugar exposure over the past two to three months, not just a snapshot from the morning you fasted for a blood draw.

That two-to-three-month window is what makes HbA1c so valuable. Fasting glucose tells you what your blood sugar is right now. HbA1c tells you what it has been doing while you were not looking. And the research is clear: even small, sustained elevations in average blood sugar, well below the threshold for a diabetes diagnosis, are linked to increased risk for heart disease, kidney damage, stroke, and early death.

How It Forms

Glucose in your bloodstream naturally sticks to hemoglobin in a process called glycation. This is not something your body does on purpose. It happens spontaneously whenever sugar and hemoglobin meet. First, glucose loosely attaches to the hemoglobin molecule. Then, through a chemical rearrangement, it locks in permanently for the life of that red blood cell. The more sugar circulating in your blood, the more hemoglobin gets glycated.

In someone with healthy blood sugar, about 5% of total hemoglobin is glycated. In someone with poorly controlled diabetes, that proportion rises because their blood is consistently sugar-rich. The test simply measures this percentage.

Heart Disease Risk

Cardiovascular risk does not suddenly appear at the diabetes threshold. It rises along a continuous gradient that begins well within the "normal" range. In the UK Biobank study of over 329,000 adults followed for a median of 11 years, risk for atherosclerotic cardiovascular disease began climbing above an HbA1c of 5.4%. The variation in heart disease risk across the full HbA1c spectrum was roughly threefold.

A Canadian population study of over 608,000 adults without diabetes confirmed the pattern: even in the prediabetes range of 5.7% to 6.4%, cardiovascular events were independently elevated. And a pooled analysis of four major U.S. cohort studies (over 20,000 adults, median follow-up of 16.7 years) found that people with HbA1c between 5.7% and 6.4% had significantly higher risk of both atherosclerotic cardiovascular disease and death from any cause compared to those below 5.7%.

What this means for you: if your HbA1c is 5.5% or above, your cardiovascular risk is already climbing, even if no one has used the word "prediabetes." Knowing this number gives you a window to act before damage accumulates.

Stroke Risk

The connection between HbA1c and stroke is especially steep in people with diabetes, but it exists even without a diabetes diagnosis. In a Swedish national registry study of over 406,000 people with type 2 diabetes, stroke risk rose in a stepwise fashion with each increment of HbA1c. Those in the highest category had roughly double the stroke risk of those at the reference level, and their likelihood of dying after a stroke was also substantially higher.

A Chinese cohort of over 11,000 adults showed that each 1-unit increase in average HbA1c over time was associated with a 12% increase in stroke risk. The top fifth of HbA1c values carried about 79% higher stroke risk compared to the bottom fifth.

Kidney Disease

Chronic kidney disease is one of the most serious complications of sustained high blood sugar, and HbA1c tracks this risk directly. In the UK Biobank, kidney disease risk began to rise independently above an HbA1c of 6.2%, with a fourfold gradient between the lowest and highest HbA1c categories. A meta-analysis of 15 studies covering over 56,000 people with type 2 diabetes found that each 1% increase in HbA1c was linked to a 12% higher risk of diabetic kidney disease, with a clear linear relationship between higher HbA1c and more kidney damage.

Cancer Associations

The link between blood sugar and cancer risk is less widely known but supported by large studies. In the UK Biobank (over 476,000 participants, average 7.1 years of follow-up), both diagnosed diabetes and HbA1c in the prediabetes range were associated with increased cancer incidence, particularly for liver, pancreatic, and endometrial cancers. A Japanese cohort study of nearly 30,000 adults found that HbA1c at the high end of the non-diabetic range (6.0% to 6.4%) was associated with higher cancer risk across all types.

The ARIC study tracked over 12,000 cancer-free participants and found a similar U-shaped pattern: both very low and elevated HbA1c were associated with higher cancer incidence and cancer death. This mirrors the mortality pattern and suggests that extremely low HbA1c may reflect underlying illness rather than optimal metabolic health.

All-Cause Mortality: The U-Shaped Curve

One of the most consistent findings across large studies is that the relationship between HbA1c and death follows a U-shape: risk is lowest in a middle range and rises at both extremes. In people without diabetes, the lowest mortality risk sits around HbA1c 5.0% to 5.4%. In people with diabetes, the nadir is around 6.5% to 7.0%.

A systematic review of 46 observational studies confirmed this pattern. In people with diabetes, the highest mortality was seen at both extremes: HbA1c above 9.0% and below 6.0%. In people without diabetes, both HbA1c above 6.0% and below 5.0% were associated with increased death risk. The very low end likely reflects frailty, malnutrition, or underlying disease rather than a harmful effect of low blood sugar itself.

Reference Ranges

HbA1c naturally rises about 0.1% per decade after age 30, even in people with normal glucose tolerance. This means a 60-year-old and a 30-year-old with identical diets and metabolic health will not have the same number. Keep this in mind when comparing your results to the tiers below.

These tiers are drawn from published research and major guidelines including the American Diabetes Association. Your lab may use different testing methods and cutpoints. Compare your results within the same lab over time for the most meaningful trend.

Tracking Your Trend

A single HbA1c reading is useful. A series of readings over time is far more powerful. In controlled research settings, the within-person variation of HbA1c is about 1.7% in healthy individuals. But in real-world primary care data from over 587,000 people, the within-person variation was roughly 20%, about three times what controlled studies show. This means a single reading can vary enough to shift your interpretation.

The practical threshold for a "real" change is about 0.5% absolute (when your HbA1c is around 7%). A shift smaller than that could simply reflect normal measurement variation rather than a true change in your blood sugar control. This is why tracking matters: two or three readings over time reveal your trajectory far more reliably than any single number.

Get a baseline reading, then retest in 3 months if you are making dietary, exercise, or medication changes. Once you reach a level you are satisfied with, retest at least every 6 months. If your goal is prevention and you are in the optimal range, annual testing is a reasonable minimum, but every 6 months gives you a tighter feedback loop.

When Results Can Be Misleading

HbA1c has a major advantage over fasting glucose: it is not affected by what you ate yesterday, whether you exercised this morning, or whether you are feeling stressed during the blood draw. But several conditions can distort the number in ways that have nothing to do with your actual blood sugar control.

• Shortened red blood cell lifespan (falsely lowers HbA1c): hemolytic anemia (a condition where red blood cells break down faster than normal), recent significant blood loss, recent blood transfusion, pregnancy in the second or third trimester, and use of drugs that stimulate red blood cell production (erythropoiesis-stimulating agents) all cause red blood cells to turn over faster, giving glucose less time to glycate hemoglobin. Your reading will look better than your actual blood sugar warrants.
• Prolonged red blood cell lifespan (falsely raises HbA1c): iron deficiency anemia, vitamin B12 deficiency, and folate deficiency extend red blood cell survival, allowing more glycation to accumulate. You may get a reading that suggests worse blood sugar control than you actually have. Correcting the nutrient deficiency may cause your HbA1c to drop without any change in your actual glucose levels.
• Hemoglobin variants: people with sickle cell trait, hemoglobin C trait, or other inherited hemoglobin variants may get falsely high or low results depending on the lab method used. If you are homozygous for any of these variants (e.g., sickle cell disease), HbA1c cannot be measured at all because you lack normal hemoglobin A. The G6PD variant (a common inherited enzyme difference), carried by about 11% of Black individuals in the U.S., is associated with a decrease in HbA1c of about 0.7% to 0.8%.
• Advanced kidney disease: multiple competing effects in kidney failure (inflammation promotes glycation while anemia shortens red blood cell life) make HbA1c unreliable. In dialysis patients, alternative markers like glycated albumin (which measures sugar attached to a different blood protein) are preferred.

Racial and ethnic differences deserve a direct note. Black individuals tend to have HbA1c values about 0.3% to 0.4% higher than White individuals at the same measured glucose levels. The reasons are partly genetic and not fully explained. Current guidelines do not adjust thresholds by race, but this gap means a Black person at 5.8% may have the same underlying glucose exposure as a White person at 5.4% to 5.5%. If your HbA1c seems inconsistent with your glucose readings, ask your lab about alternative markers.