A1c shows how much sugar your blood has carried over time, not just on the morning of the test. The higher it runs, the more heart disease and stroke are seen, and that rise begins below the level that defines diabetes.
On its own, though, A1c is a weak predictor of who will have a heart attack. Its real value is in flagging prediabetes and diabetes, because those diagnoses change how firmly everything else should be managed.
Key takeaways
- Under 5.7% is normal, 5.7% to 6.4% is prediabetes, and 6.5% or more points to diabetes.
- Heart risk climbs steadily with A1c and is roughly double once diabetes is present.
- Prediabetes raises risk too, but the added risk for any one person is small.
- Adding A1c to the usual risk factors improves heart risk prediction only slightly.
- Anemia, blood loss and some hemoglobin traits can make an A1c result misleading.
What A1c measures
Glucose in your blood sticks to hemoglobin, the protein inside red blood cells. A1c is the share of hemoglobin that has glucose attached. Because red cells circulate for a long time, the result reflects your glucose exposure over time.
That makes it steadier than a fasting glucose reading. Glucose levels move around within a day and from one day to the next, while A1c changes slowly.
The A1c ranges
| A1c | Category | What it usually means |
|---|---|---|
| Below 5.7% | Normal | No sign of raised blood sugar |
| 5.7% to 6.4% | Prediabetes | Blood sugar is above normal. Often reversible |
| 6.5% or higher | Diabetes range | Needs a confirming test and a treatment plan |
In the United States, prediabetes is defined by an A1c of 5.7% to 6.4%, and some international bodies use 6.0% to 6.4% instead.1 An international expert committee set 6.5% or higher as the level for diagnosing diabetes and advised confirming it with a repeat test.2
Prediabetes is common. It affects about 1 in 3 US adults, so a result in that range puts you in very large company.
How A1c relates to heart risk
The relationship is graded. There is no single line where risk switches on.
A US study followed 11,092 adults who had no diabetes and no heart disease at the start. Compared with people whose A1c was 5.0% to under 5.5%, coronary heart disease risk was 23% higher at 5.5% to under 6.0%, 78% higher at 6.0% to under 6.5%, and 95% higher at 6.5% or above.3
Once diabetes is established, the picture is clearer still. In a pooled analysis of 102 studies and 698,782 people, diabetes roughly doubled the risk of coronary heart disease and of ischemic stroke, after accounting for age, sex, smoking, blood pressure and body mass index.4
Prediabetes sits in between. A review of 129 studies covering more than 10 million people found that prediabetes was linked with a 15% higher risk of cardiovascular disease than normal blood sugar.5
Keep that last figure in proportion. In absolute terms the difference was about 9 extra cardiovascular events per 10,000 people per year. Prediabetes matters mostly because of where it leads and what tends to travel with it.
What A1c does not tell you
A1c is not a heart risk test in its own right. When researchers pooled 73 studies of nearly 295,000 people without diabetes or heart disease, adding A1c to conventional risk factors gave little extra benefit in predicting cardiovascular events.6
That same analysis found a J-shaped curve, meaning the lowest A1c values were not the safest. Lower is not always better.
There are also situations where the number itself is unreliable:
- Hemolytic anemia, major blood loss or a recent transfusion, all of which change how long red cells last
- Certain inherited hemoglobin traits, which interfere with some lab methods
The expert committee that set the diagnostic level warned that any condition that changes red cell turnover will give a false A1c result.2 If one of these applies to you, tell your clinician, because a glucose-based test may be needed.
A1c also says little about what happens earlier. Blood sugar can look normal for years while insulin runs high to keep it there. We cover that on our page about insulin resistance.
Why a diabetes-range result changes the heart plan
Diabetes is one of the inputs to the PREVENT risk calculator, and A1c can be added as an optional input. A result in the diabetes range moves your estimated risk up.
It also changes how other tests are read. Under the 2026 US cholesterol guideline, a calcium score of zero supports holding off on medication for many people, but not in the same way for those with diabetes. Our guide to diabetes and heart disease risk goes into this.
For prediabetes the news is better. In trials, intensive lifestyle change reduced new cases of diabetes by about 6 per 100 people per year over three years.1 See prediabetes and heart risk for what is reversible.
A1c in a Cardio Res-Q assessment
A1c is part of the lab panel in both Cardio Res-Q assessments, measured together with fasting insulin and glucose. The Comprehensive Assessment ($775) includes these with ApoB, Lp(a), hs-CRP, homocysteine, the Cleveland Heart cardiac panel and Dr. Varano's clinical evaluation. The Enhanced Assessment ($999) adds a CIMT ultrasound and a coronary calcium score.
Having insulin, glucose and A1c side by side shows more than any one of them alone. You fast for 10 to 12 hours before the draw. For the full sequence, read what to expect at your assessment, or go to the Cardio Res-Q program page to book.
Are You a Candidate for Cardio Res-Q?
Three quick questions about the risk factors we screen for. Your answers stay between you and the clinic.
Frequently asked questions
My A1c is in the prediabetes range. Will I get diabetes?
Not necessarily. In the US, roughly 1 in 10 people with prediabetes go on to diabetes each year, which means most do not in any given year. Weight loss and regular activity lower the odds further.
Can my A1c be normal and my heart risk still be high?
Yes. A1c covers one risk factor. Blood pressure, ApoB, Lp(a), smoking and family history are separate, and any of them can be high when blood sugar is normal.
Is an A1c of 5.9% dangerous?
It is in the prediabetes range, not an emergency. It is a signal to look at weight, activity, sleep and the rest of your risk factors, and to recheck over time as your clinician advises.
Why would my A1c and my glucose disagree?
They measure different things: one a single moment, the other a long stretch. Anemia, recent blood loss and some hemoglobin traits can also skew A1c. Your clinician can sort out which result to trust.
Related reading
This article is for general education and is not medical advice. Decisions about testing and medication should be made with your clinician. If you have chest pain or other symptoms of a heart attack, call 911.
