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Medically reviewed by Kelly Wood, MD — Written by Erica Manfred — Updated on February 25, 2025

About A1C testing
Levels
Calculating A1C
Accuracy
How often
What to do about high levels
Takeaway

A1C levels refer to the amount of hemoglobin in your blood that has glucose attached to it, which can be useful for monitoring blood sugar levels over time.

Managing your blood sugars is a key part of your overall diabetes care plan. While using a fingerstick meter or a continuous monitor can be part of your daily routine, they only provide a snapshot of your blood sugar at that moment in time.

The hemoglobin A1C test, however, offers a more complete picture of your diabetes management over the past few months.

They may better reflect how your glucose levels fluctuate, whether it’s a 3 a.m. low or a dramatic spike after lunch, and what that means overall for how you can change your care plan with your healthcare team’s help.

What does the A1C test for diabetes measure?

An A1C test measures the amount of hemoglobin in the blood that has glucose attached to it. Hemoglobin is a protein found inside red blood cells that carries oxygen to the body. Hemoglobin cells are constantly dying and regenerating.

Their lifespan is roughly 3 months, meaning the A1C test offers an average of your blood sugars over that period.

That result isn’t meant to mirror your average glucose readings on any given day, but it provides your doctor a picture of how effective your blood sugar management has been.

History of the A1C in diabetes care

A1C tests became available in the 1980s and quickly became an important tool in monitoring diabetes control.

A1C tests measure average blood glucose over the past 2 to 3 months. So even if you have a high fasting blood sugar, your overall blood sugar may be normal, or vice versa. A normal fasting blood sugar may not eliminate the possibility of type 2 diabetes.

This is why A1C tests are often used to diagnose and screen for prediabetes and diabetes. Because they don’t require fasting, the test can be given at any time as part of an overall blood screening.

About A1C levels

Someone without diabetes will have about 5% of their hemoglobin glycated.

If there’s too much glucose attached to the hemoglobin cells, you’ll have a higher A1C.

Higher A1C results may indicate prediabetes, type 1 or 2 diabetes, or another type of diabetes or health condition.

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Calculating your A1C level

The American Diabetes Association provides a calculator that shows how A1C levels correlate to glucose levels.

Importantly, glucose levels do not always match an A1C. Neither are precise but are meant to be data points to use along with other information in your overall diabetes care plan.

Your healthcare team can best guide you on what A1C levels mean, how that relates to blood sugars, and what you may need to consider for your diabetes plan.

How accurate are A1C results?

Accuracy is relative when it comes to A1C or even blood glucose checks.

The A1C result may vary based on different factors, including those with kidney failure, liver disease, or severe anemia.

Your result could be up to half a percent higher or lower than the actual percentage. That means if your A1C is 6%, it might indicate a range from 5.5% to 6.5%.

Before confirming a diagnosis of diabetes, your doctor may repeat the test on a different day.

What can affect A1C results?

Read more about different factors, including medications and health conditions, that may affect your A1C levels. This can include people with diabetes, who may experience blood sugar swings that may impact the broader averages.

How often do you need an A1C test?

For many people with diabetes, your healthcare team may order an A1C every 3 months. They may want more frequent measurements in certain cases, including:

if you are 45 years or older and have a family history or other diabetes risk factors
your diabetes treatment is being adjusted
you and your doctor are setting certain blood sugar targets
you’re pregnant or have been diagnosed with gestational diabetes
other diabetes-related concerns that are part of your care plan

What to do if levels are high

High A1C levels over time may raise your risk of developing diabetes-related complications, including:

cardiovascular diseases, like stroke and heart attack
kidney disease
nerve damage
eye damage that may result in blindness
numbness, tingling, and lack of sensation in the feet due to nerve damage
slower wound healing and infection

For those managing any type of diabetes or facing risk for the condition, you can address blood sugar and A1C levels by working with your healthcare team on several fronts: eating habits, physical activity, weight management, mental health, and other healthcare.

You can work with your doctor and healthcare team to develop a diabetes care plan, whether you’re diagnosed with prediabetes, type 1 or 2 diabetes, or gestational diabetes. They may also want to discuss specific treatments if you face higher risk for developing these conditions.

The takeaway

The A1C test measures the amount of hemoglobin in the blood with glucose attached. The test provides an average of your blood sugar readings for the past 3 months.

It monitors blood sugar levels and diagnoses and screens for prediabetes and diabetes. People with diabetes should take an A1C test at least twice a year and more frequently in some cases.

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SOURCES:

A1C test for diabetes and prediabetes. (2024). https://www.cdc.gov/diabetes/diabetes-testing/prediabetes-a1c-test.html

Diabetes, heart disease and stroke. (2021). https://www.niddk.nih.gov/health-information/diabetes/overview/preventing-problems/heart-disease-stroke

Diagnosis and classification of diabetes: Standards of Care — 2025. (2024). https://diabetesjournals.org/care/article/48/Supplement_1/S27/157566/2-Diagnosis-and-Classification-of-Diabetes

Eyth E, et al. (2023). Hemoglobin A1C. https://www.ncbi.nlm.nih.gov/books/NBK549816/

The A1C test and diabetes. (2018). https://www.niddk.nih.gov/health-information/diagnostic-tests/a1c-test?dkrd=/health-information/diabetes/overview/tests-diagnosis/a1c-test

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Medically reviewed by Kelly Wood, MD — Written by Erica Manfred — Updated on February 25, 2025

All About The Hemoglobin A1C Test - Healthline

What does the A1C test for diabetes measure?

History of the A1C in diabetes care

About A1C levels

Calculating your A1C level

How accurate are A1C results?

What can affect A1C results?

How often do you need an A1C test?

What to do if levels are high

The takeaway

How we reviewed this article:

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