You do not need to fast for an A1C test. You can usually eat and drink normally before having your blood drawn, because the A1C test reflects your average blood glucose over approximately the previous 2 to 3 months rather than your glucose level at one particular moment.
A1C measures the percentage of hemoglobin in your red blood cells that has glucose attached to it. Because red blood cells circulate for many weeks, the result gives us a longer-term picture of your glucose rather than showing only what your blood sugar is at the moment the sample is taken.
This is why eating breakfast shortly before the test does not normally cause the kind of immediate change in A1C that it would cause in a regular blood glucose measurement.
If you want to understand how the result relates to estimated average glucose, you can also use our HbA1c to estimated average glucose calculator.
This is where confusion often happens. Your A1C may be drawn at the same appointment as other laboratory tests, and some of those tests may require fasting.
For example, a fasting plasma glucose test requires fasting for at least 8 hours, with only water allowed. Some cholesterol or triglyceride testing may also be ordered with fasting instructions depending on the test and your clinician's request. :chatgpt-content-reference{index="2"}
If you are having only an A1C test, the test itself generally does not require you to stop or delay your usual diabetes medication.
However, if you have been instructed to fast because another test is being performed at the same time, ask your healthcare professional how to handle insulin or other glucose-lowering medication during the fasting period. Do not independently skip or change a prescribed dose.
For diagnosing diabetes in most nonpregnant adults:
When there are no clear symptoms of diabetes, an abnormal diagnostic result usually needs confirmation with a repeat A1C or another accepted diabetes test. :chatgpt-content-reference{index="3"}
For someone who already has diabetes, an A1C result is used differently: it helps assess longer-term glucose management. A treatment goal is individualized rather than being the same for every person.
You can learn more about interpreting the test on our page explaining A1C testing and levels.
Yes. A1C is very useful, but certain conditions can make it less reliable. Examples include some hemoglobin variants, recent blood loss or transfusion, some types of anemia, kidney disease and other conditions that alter red blood cells. :chatgpt-content-reference{index="4"}
If your A1C result does not seem to match your home glucose readings or CGM results, discuss the discrepancy with your healthcare professional rather than assuming one result must be correct.
If you are having an A1C test only, you do not need to fast. You can generally eat and drink normally before the test.
If your clinician has ordered fasting glucose, certain lipid tests or another fasting blood test at the same appointment, follow the fasting instructions for those tests instead.
All the best,
Dr. Albana Greca, MD, MMedSc
Family Physician, IDF Fellow
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