Doctor explaining a patient on a tablet how A1C is calculated

How is A1C Calculated?

If your doctor has told you your A1C or you’ve seen it in your lab work, you might wonder what it is, what the number means, and how A1C is calculated. This post will explain everything you need to know about A1C.

Doctor explaining a patient on a tablet how A1C is calculated

What is A1C

Hemoglobin A1C is a simple blood test that measures your average blood sugar levels over the past 2-3 months. If you are at risk for diabetes or have prediabetes, your doctor may do this test at your annual physical, but if you have uncontrolled type 1 diabetes or type 2 diabetes, your doctor may order this test as often as every 3 to 6 months. 

A1C is one of the diagnostic tests used to diagnose prediabetes and diabetes. Your doctor may also use fasting blood sugar, glucose tolerance tests, and random blood sugar tests to make an accurate diagnosis. 

How A1C is calculated

After you eat carbohydrates, they’re broken down into sugars that enter your bloodstream. This sugar attaches to hemoglobin, a protein found in red blood cells. The higher your blood sugar is, the more sugar will be attached to the hemoglobin. 

An A1C test checks the percentage of hemoglobin that’s coated in sugar. Since red blood cells are replaced every 2-3 months in the body, it shows us the average percentage over the life of the red blood cells and can be done as often as every 3 months. 

What do the results mean?

Blood test results and blood in tubes

Testing your hemoglobin A1C can give your doctor and care team important information to modify your treatment plan and help you avoid complications of uncontrolled high blood sugars. 

While your A1C provides useful information, it’s not the only number used in diagnosing and managing your diabetes. If someone has high and low blood sugars, their A1C could appear normal while their diabetes is still uncontrolled. Because of this, A1C is often used along with other labs like your fasting blood glucose (FBG) and postprandial (post-meal) blood sugars for a big picture of what your blood sugars are doing.

Here’s what the A1C results can mean: 

HbA1C Meaning

Less than 5.7%Normal
5.7% to 6.4%Pre-diabetes
6.5% or aboveDiabetes

But what does that mean in terms of actual blood sugar values? To know how the percentages relate to estimated average glucose (EAR) use the chart below.

A1C and Estimated Average Glucose

7%154 mg/dL
8%183 mg/dL
9%212 mg/dL
10%240 mg/dL

A1C vs Fasting Blood Glucose 

While A1C tells us the average blood sugar over 2 to 3 months, a fasting blood sugar can quickly alert your doctor that something could be wrong. This number tells us what your blood sugar is when you have not eaten any carbohydrates for the past 8-12 hours.

A fasting blood glucose is included in most routine lab work. If this number is elevated, your doctor may request an A1C to get a better picture of what you’re blood sugars have been over the previous couple of months. 

Here are a couple of charts to help you see how fasting blood sugar and A1C relate to each other, per the American Diabetes Association (note that other professional organizations may have slightly different values for diagnostic criteria). 

Woman testing her blood sugar with a glucose monitor

Fasting Blood Glucose Meaning

Fasting Blood SugarDiagnosis
Under 100 mg/dLNormal
100-125 mg/dLPrediabetes
Over 125 mg/dLDiabetes

How to improve Hemoglobin A1C

How you improve your A1C depends on your diagnosis and treatment plan from your doctor and diabetes care team. If you have prediabetes, your doctor may suggest that you first try adjusting your diet and exercise routine to help naturally reverse prediabetes

If you have diabetes, you may be on blood sugar-lowering medications that will help reduce your A1C. In this case, especially if you are taking insulin, work with your diabetes care team to develop a diet and exercise plan that will work alongside your medications to lower your blood sugars and A1C safely. 

Here are things you can do to help lower your A1C:

  • Follow a healthy controlled carbohydrate diet.*
  • Include mild to moderate exercise in your daily routine.*
  • Develop healthy sleep habits to improve sleep quality. 
  • Manage stress levels.
  • Take your medications as directed. 

*If you are on blood sugar-lowering medications or insulin, consult your doctor and diabetes care team before any new diet or exercise changes. 

The bottom line

Hemoglobin A1C is a lab that can tell you and your doctor what your average blood sugars have been over the past two to three months. It is used as one of the diagnostic tests to diagnose prediabetes and diabetes. If your A1C is higher than it should be, you and your diabetes care team can devise a goal and steps to reach your goal by adjusting your diet, exercise, sleep, stress, and taking your medications as directed.

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