Why Your Average Blood Sugar Might Not Match Your A1C (2024)

Updated 8/11/22

Read time: 7 minutes

• The relationship between average blood sugar and A1C isn’t as clear as many of us think. For example, you might have a normal A1C, but high fasting glucose.
• One way to increase how closely your average blood sugar aligns with your A1C is to check your blood sugar levels more regularly.
• Individual variations in hemoglobin and differences in red blood cell turnover can impact A1C results.

Youtest your meter for accuracy and everything looks good. You take your average bloodsugar and convert it to A1C using a table, calculator, or equation you find online. Then, you get your blood workresults and learn that your actual A1C is not even close to what you were estimating.

What is the deal?It turns out, the relationship betweenaverageblood sugarandA1C isn't as clear as most of us think.

Here are somereasons why someone'sactual A1C maybe higher or lower than expected or why you might have a normal A1C, but high fasting glucose.

Why Use A1C?

First, let's briefly go over why A1C is used to approximate averageblood sugar overtwo to three months:

• As glucose enters your blood, it attaches to a protein in your red blood cells called "hemoglobin."
• Hemoglobin is the same protein that carries oxygen in your bloodstream. It's what gives blood itsred color
• A1Cmeasuresthepercent of red blood cells with glucose attached to the hemoglobin.

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The Ultimate Guide to A1C

Why Doesn't MyA1C Match My Average Blood Sugar?

1.A glucose meter averagedoes not usually reflect the average over a full 24 hours

If you are not on a CGM, it's tough to get a full picture of your average bloodsugar throughout the day. We generallycheck much more during the day thanatnight, and nighttime glucose values may be very different from daytime values.

Wealso tend to test more often before eating(whenblood sugar levelsare typically lower), and less oftenafter meals (whenblood sugar levels aretypically higher).

So, for many people,a blood glucose meter average doesn'taccuratelyreflect average blood sugarover a full 24 hours.A1C, on the other hand, does.

If you want your meter average to better reflect your A1C values, check more often. And make sure you check at various times throughout the day, includingone to two hours after eating.And be sure you are comparing the same timeframes measured in your glucose meter reports to the timeframe your A1C is measuring (i.e. the three months prior to the date of your A1C test).

2. The correlation between A1C and estimated average blood glucose is not perfect.

Most (if not all) averageblood sugar to A1C conversiontables and calculators use the below equation to estimate average blood glucose from A1C:

Estimated Average BG (mg/dL) = 28.7 X A1C (%) - 46.7

This equation is based on data froma study of over 500 subjects (268 T1Ds, 159 T2Ds, and 80 non-diabetics)at 10 internationalcenters aroundthe world. The A1C values were measured in a central laboratory, sodifferences in laboratory method or technique were not a factor.

People were studied for 12 weeks, with two days of CGM and three days of 7-point glucose profiles each week. The BG meters used were carefully standardized and calibrated.

The graph below shows the data used to derive the relationship between average glucose and A1C. As you can see, there is a lot of scatter.

A number ofdata points are offthe trend line by ± 1%. For some A1C values,the spread is more significant. Notice the range ofA1Cs for people with an average glucose of ~110 mg/dL—it goesfrom below 4% to almost 9%.

Importantly the study concluded that the equation could be used to convertA1C to estimated average blood glucose values for "most patients." Notallpatients.

Results of a study of 507 subjects. Published in Diabetes Care 31:1473-1478, 2008.

It should be noted that this formula was meant to provide an estimated average glucose based off a lab-calculated A1C. It wasn’t created to work in reverse by providing an estimated A1C based off average glucose readings from periodic at-home blood sugar checks. As previously stated, the average blood sugar on your glucose meter likely doesn’t accurately reflect your true, 24-hour average blood sugar.This is why youmight have a normal A1C, but high fasting glucose.

3. Hemoglobin and red blood cell characteristics can vary from person to person.

Genetics can impact both the type and behavior of hemoglobin. Some people have less common types of hemoglobin, calledhemoglobin variants. Although most A1C tests in the U.S. now account for the most common types of hemoglobin variants, there are still variants that can impact accuracy of some A1C tests.

The rate that glucose attaches to hemoglobin may also impact A1C accuracy.

For some people, glucose attaches to hemoglobinvery quickly,which may cause an A1C result to be higher than expected when compared with true average blood sugar.

For others, glucose attaches to hemoglobinmore slowlyand may cause an A1C result to be lower than expected.

4. The rate of red blood cell turnover variesfrom person to person.

Anemia, some blood disorders, advanced liver disease and kidney disease, spleen damage and pregnancy can all cause changes to red blood cells that impact A1C accuracy.Certain medical treatments like some opioid medications and HIV medication can also make A1C results less aligned with actual average blood sugar.

Other Factors Related to A1C Levels May Play a Role

Instudies, people whogive blood,have any internal bleeding, blood loss, or have red blood cell transfusions may have either falsely elevated or decreased A1Cs.

Chronic alcohol intakes, lead poisoning, and even certain vitamin supplements are alsobeing investigatedfor potential impacts on A1C.

It's important to note here that A1C is not a perfectpredictor of your risk ofdiabetes complications. It only predicts your complication risk if it accurately representsyour true average blood glucose.

Bottom Line About Average BloodSugar and A1C