- The A1c has been the gold standard diabetes test for decades, but certain health conditions, medications, and genetics can make it read falsely high or falsely low.
- Tests like the oral glucose tolerance test, fructosamine test, and glycated albumin test can be used if your healthcare provider thinks your A1c results may not accurately reflect your average glucose.
- Using a continuous glucose monitor is the best indicator of average glucose, since it takes hundreds of readings per day and there aren’t interfering factors like age, genetics, and other health conditions.
- The A1c is still a very good test for most people, but it’s helpful to understand the circumstances that can affect A1c results so that you and your doctor can determine if other testing methods might be needed.
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We rely on our A1c test to let us know how we’re doing with diabetes, and our doctors rely on it to inform treatment decisions. But what if the one test that’s supposed to be the all-knowing indicator of our diabetes status is…well, inaccurate?
The A1c is an excellent test for most people, but it’s not perfect. Certain health conditions, medications, and even your own genetics can make the result falsely high or falsely low. In fact, the A1c can sometimes be off by as much as 1%! That means your doctor could think your diabetes is doing better (or worse!) than it actually is.
This isn’t a reason to question the results of every lab test – it’s just important to know that there are situations when your A1c might not accurately reflect your blood sugar.
How Does an A1c Test Work?
Believe it or not, the A1c test doesn’t actually measure blood sugar – it measures how much glucose has attached itself to your red blood cells.
Red blood cells live for about 120 days. During that time, glucose naturally sticks to hemoglobin, which is a protein in red blood cells. When there’s more glucose circulating in your blood, more of it binds to the hemoglobin. So anything that changes how long your red blood cells live or how they function can affect your A1c.
Here are 10 situations that can make your A1c test inaccurate:
1. Anemia
Anemia is one of the most common reasons your A1c might not accurately reflect your blood sugar. How it affects blood sugar depends on the type of anemia you have.
Iron deficiency anemia can cause your A1c to appear higher than it really is. When your body doesn’t have enough iron, your red blood cells may live longer than usual, which gives them more time to accumulate glucose.
On the flip side, other types of anemia (such as hemolytic anemia) can cause red blood cells to break down faster than normal. Since these cells don’t live long enough to collect as much glucose, your A1c may appear falsely low.
2. Blood Loss or Blood Transfusion
If you lose a large amount of blood from an injury, surgery, or another medical condition, your body will start to make new red blood cells. Because the new cells haven’t been exposed to glucose for very long, it can lead to a falsely low A1c.
If you’ve recently had a blood transfusion, the red blood cells you received from a donor may be older or newer than your own, which can make your A1c appear higher or lower. Blood transfusions typically lower A1c because donor cells usually come from people without diabetes.
Now that you know that blood loss can contribute to a falsely low A1c, you can donate a ton of blood a week before your next doctor’s appointment and your A1c result will be great! We’re kidding…do NOT do that!
3. Surgery and/or Steroids
Falsely elevated hemoglobin A1c can sometimes be caused by surgery or steroids. Surgery puts stress on your body, which can trigger the release of stress hormones that raise blood sugar. Medications like steroids (which are often used before, during, or after surgery) can also cause blood sugar to rise. Glucose levels can be higher than normal for several days (sometimes weeks) after a surgical procedure.
4. Chronic Kidney Disease
Chronic kidney disease (CKD) can affect how long red blood cells survive. Most often, CKD causes an artificially low A1c because red blood cells break down faster than normal, leaving them with less time for glucose to attach to hemoglobin.
However, CKD can sometimes lead to an artificially high A1c. Iron deficiency, which is common in people with CKD, can cause red blood cells to live longer or alter the mix of red blood cells in circulation, giving glucose more time to attach to hemoglobin.
5. Dialysis
Many people on dialysis have anemia, which we’ve already learned can affect A1c results. Some people on dialysis may also receive medications such as erythropoietin (EPO), which stimulates the production of new red blood cells. Because newer cells haven’t been exposed to glucose as long, it can make the A1c appear falsely low.
6. Liver Disease
Advanced liver disease can cause anemia, bleeding, or an enlarged spleen that removes red blood cells from circulation more quickly. All of these can shorten the average lifespan of red blood cells and make the A1c appear falsely low. Depending on the specific liver condition, however, A1c can occasionally be misleading in either direction.
7. Pregnancy
Pregnancy speeds up red blood cell turnover (particularly during the second and third trimesters), which means A1c levels in pregnancy often read lower than they otherwise would. A1c also can’t show the after-meal glucose spikes that are especially important during pregnancy. For these reasons, fingersticks and CGM readings are usually more helpful for day-to-day treatment decisions during pregnancy.
8. Certain Inherited Blood Disorders
Some inherited blood disorders can affect the accuracy of the A1c. Sickle cell disease causes red blood cells to break down faster than normal, and thalassemia affects how the body makes hemoglobin. Both of these conditions can lead to a falsely low or falsely elevated hemoglobin A1c. Your doctor can determine whether an A1c test is appropriate or whether another method of measuring blood sugar would be more effective.
9. Genetics and Ethnic Background
Some people inherit different types of hemoglobin. This is more common in people with ancestry from regions such as:
- Africa
- The Mediterranean
- South and Southeast Asia
- The Middle East
Some A1c tests can be affected by these differences, which means the result may not always accurately reflect a person’s average blood sugar. Also, some people naturally have slightly higher or lower A1cs than others, simply due to genetics.
10. Older Age
As we get older (usually around 65-ish, but everyone’s different), A1cs can naturally start to creep up, even if blood sugar hasn’t changed much. As you get older, your doctor may set a slightly higher A1c goal for you, depending on your overall health and your risk for hypoglycemia.
Frequently Asked Questions: How Accurate Is Your A1c?
🧪 If the A1c isn’t always accurate, what tests can be used instead?
Continuous glucose monitoring and time in range are really the best ways to measure average blood glucose, but other testing options are available:
- Fasting blood glucose: As the name implies, this measures your blood glucose after a period of fasting (usually 8-12 hours).
- Fingerstick glucose monitoring: Also measures the blood glucose at a single point in time, but can help show patterns when checked at useful times.
- Oral glucose tolerance test: This measures how your body responds to a sugary drink.
- Fructosamine test: This measures glycated proteins in your blood to estimate average glucose over the last two to three weeks, and is not affected by red blood cell lifespan.
- Glycated albumin test: This measures the amount of glucose attached to albumin (a protein in your blood), reflecting blood sugar over the last two to three weeks.
Your doctor will determine the best option for you based on your individual circumstances and health history.
🔍 What’s the difference between the A1c test and the glucose management indicator (GMI) on my CGM?
A1c and GMI estimate glucose in different ways. The A1c measures the amount of glucose attached to hemoglobin in your red blood cells. The GMI estimates your average blood sugar based on glucose readings from your CGM. GMI is not affected by things like anemia, kidney disease, or changes in red blood cell lifespan.
This is why we agree with Dr. Ian Blumer’s proposal to replace the term “GMI” with “sensor-derived A1c”…because that’s what it is!
✌️ Can people with type 2 diabetes get CGMs through insurance?
If you have type 2 diabetes, most insurance plans cover a CGM if you are on at least one shot of insulin per day. If you aren’t able to get a CGM through insurance, you can purchase one over-the-counter without a prescription (or ask your doctor’s office if they have any free samples) and use it for two weeks. Even wearing a CGM for a short period of time can give you a good idea of your actual average blood sugar.
⁉️ If you don’t use a CGM, how would you ever know if your A1c is inaccurate?
That’s an excellent question! You can plug your A1c into the American Diabetes Association’s online calculator to determine your approximate average blood sugar.
For example, if your A1c is 7.4, that means your average blood sugar is 166 mg/dL. However, if you notice that you’re frequently lower or higher than 166 mg/dL, your doctor may recommend an alternative way to test your average glucose.
💊 Are there any vitamins that lower A1c?
Vitamins and supplements are not regulated by the FDA, and the American Diabetes Association has stated that “supplements are not proven as an effective option for lowering your blood glucose.” The ADA’s Standards of Care do not recommend taking supplements unless you have been diagnosed with a nutrient deficiency, so always discuss any vitamins and supplements you take or want to take with your healthcare team.
How Accurate Is Your A1c? Key Takeaways
The A1c is still a very good test for most people with diabetes. But it’s important to understand the conditions and circumstances that can affect A1c results so that you and your doctor can determine if other testing methods might be needed.
In a perfect world, everyone with diabetes (and even prediabetes) would have access to a CGM, because the GMI (or sensor-derived A1c…still hoping that name sticks!) is the best indicator of average glucose, since there aren’t any interfering factors. And instead of giving you just one average, a CGM shows the highs, lows, and everything in between.
If your A1c result doesn’t seem right, don’t be afraid to ask whether it’s telling the truth, the whole truth, and nothing but the truth!
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