What is the A1C test for diabetes?

The A1C test is a test that measures a person's average blood glucose level over the past 2 to 3 months. Hemoglobin is the part of a red blood cell that carries oxygen to the cells and sometimes joins with the glucose in the bloodstream. Also called hemoglobin A1C or glycosylated hemoglobin, the test shows the amount of glucose that sticks to the red blood cell, which is proportional to the amount of glucose in the blood.

Hemoglobin A1C (A1C) test results reflect average glycemia over several months and correlate with mean plasma glucose levels. An estimated average glucose (eAG) is directly related to A1C and also provides an individual with an estimate of average blood glucose over a period of time, but it uses the same values and units that are observed when using a glucose meter or recording a fasting glucose value on a lab report.

If you have diabetes, at least every 3 to 6 months, your doctor should order an A1C test, which may be reported as eAG. It is a different number than your blood glucose checks. The result will tell you your average blood glucose for the past 2 to 3 months.

Although the A1C test is an important tool, it can't replace daily self-testing of blood glucose for those who need it. A1C tests don't measure your day-to-day control. You can't adjust your insulin on the basis of your A1C tests. That's why your blood sugar checks and your log of results are so important to staying in effective control.

Dr. Jack Merendino, MD

The hemoglobin A1c (many people just say “A1c”) is a blood test that reflects average blood sugar levels in your blood stream over the preceding two to three months. The higher your average blood sugar levels have been during this time, the higher the hemoglobin A1c levels are. People who do not have diabetes typically have hemoglobin A1c levels under 5.7. Someone with very poorly controlled blood sugar may have a value over 10.

Several major studies have shown that people with hemoglobin A1c levels close to the normal range will have fewer problems related to diabetes over time, especially where problems with the eyes, kidneys and nerves are concerned. Therefore, people are often told that they should strive to get their hemoglobin A1c under 7. That’s usually a good target, but there are exceptions. 

Everyone knows that very low blood sugars (hypoglycemia) are dangerous. The lower your average blood sugar level is, the more likely you are to have episodes of a low blood sugar. Therefore, for most people the risk of low sugar reactions is what limits how far they can lower their hemoglobin A1c. For example, several other studies have shown a higher mortality for people who have hemoglobin A1c levels that are too low. In these studies, having a value in the low- to mid-5 range appeared more dangerous than having values in the 6-range. People who are at high risk of low blood sugar reactions, or those who have less concern about long-term complications, may have targets that are higher than 7. An example might be a person who really does not feel the symptoms of a low blood sugar, or a person in his or her 80s who needs to be more concerned about short-term low sugar reactions rather than complications that will develop over many years. On the other hand, if some is being treated with diet only, or is taking only metformin, which does not cause hypoglycemia on its own, it is very safe for them to have hemoglobin A1c levels in the 5 range. 

One more very important point:  the hemoglobin A1c represents an average, but fluctuations in the blood sugar are also very important. It’s generally safer to have blood sugars that don’t vary widely. For my patients I would rather have a hemoglobin A1c that is a touch higher without too many peaks and valleys than to have a lower value with wide swings all the time. Best of both worlds: numbers that are close to normal and don’t jump around too much.

Mrs. Marjorie Nolan Cohn
Nutrition & Dietetics Specialist

The A1C test is used to diagnose diabetes. The higher your A1C level, the higher your risk of diabetes complications. An A1C level of 6.5 percent or higher on two separate tests indicates you have diabetes. A result between 5.7 and 6.4 percent is considered prediabetes, which indicates a high risk of developing diabetes.

For someone who doesn't have diabetes, a normal A1C level can range from 4.5 to 6 percent. Someone who's had uncontrolled diabetes for a long time might have an A1C level above 9 percent.

It's important to note that the effectiveness of A1C tests may be limited in certain cases. For example:

  • If you experience heavy or chronic bleeding, your hemoglobin stores may be depleted. This may make your A1C test results falsely low.
  • If you don't have enough iron in your bloodstream, your A1C test results may be falsely high.
  • Most people have only one type of hemoglobin, called hemoglobin A. If you have an uncommon form of hemoglobin (known as a hemoglobin variant), your A1C test result may be falsely high or falsely low. Hemoglobin variants are most often found in blacks and people of Mediterranean or Southeast Asian heritage. Hemoglobin variants can be confirmed with lab tests. If you're diagnosed with a hemoglobin variant, your A1C tests may need to be done at a specialized lab for the most accurate results.

Also keep in mind that the normal range for A1C results may vary somewhat among labs. If you consult a new doctor or use a different lab, it's important to consider this possible variation when interpreting your A1C test results.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.