A1C test: what it measures and how it works

What the A1C test measures

Sugar in your blood sticks to hemoglobin, the oxygen-carrying protein in red blood cells. It stays attached for the life of the cell, about three months. The more sugar in your blood over that time, the higher the percentage of coated hemoglobin.

That is why A1C is a long-term average. It does not show daily highs and lows, and it does not replace finger-stick or CGM checks for people who manage diabetes day to day. Your lab may also report an estimated average glucose (eAG), which converts A1C into mg/dL, the unit glucose meters use.

For a deeper look at the marker, see the hemoglobin A1c lab guide.

Do you need to fast?

No. A1C does not require fasting, so the blood can be drawn at any time of day. If it is drawn with a fasting glucose or a lipid panel, follow the fasting instructions for those tests.

Tell your clinician about medicines you take and if you are pregnant. Do not stop a medicine unless your clinician tells you to.

Home A1C tests vs a lab test

Home options come in two kinds: finger-stick kits you mail to a lab, and quick point-of-care devices. They can be useful for tracking, with limits:

• Monitoring, not diagnosis. The FDA clearance for one mail-in home A1C kit says it is for tracking long-term blood sugar in people with diabetes and is not to be used to diagnose or screen for diabetes. NIDDK says point-of-care tests should not be used for diagnosis.
• Some people should not rely on them. Hemoglobin variants such as hemoglobin S or C, and conditions that shorten red cell life, such as blood loss or hemolysis, can make results unreliable.
• Confirm with a lab. If a home result is 5.7% or higher, get a lab A1C or another lab glucose test before drawing conclusions.

What can make an A1C result wrong

Anything that changes how long red blood cells live can push A1C away from your true average:

• Falsely high: severe iron deficiency.
• Falsely low or unreliable: recent blood loss or transfusion, hemolysis, sickle cell disease, erythropoietin treatment and hemodialysis.
• Other: kidney failure, liver disease and hemoglobin variants, which are more common in people of African, Mediterranean or Southeast Asian descent.
• Not used for: diagnosing gestational diabetes or type 1 diabetes.

If your A1C does not match your glucose readings, ask about a ferritin and CBC check, or a different glucose test. A CGM-based estimate is covered in GMI vs A1C.

How often to test

• Adults 35 and older, and younger adults with risk factors such as excess weight, prediabetes or PCOS: testing is recommended. With a normal result, your clinician sets the schedule.
• Prediabetes: usually every 1 to 2 years.
• Diabetes: at least twice a year, and more often if you are not meeting your goals or your treatment changed.

A1C is a slow average. If you have symptoms of very high blood sugar, such as strong thirst, frequent urination, blurry vision or unusual tiredness, don't wait for a lab result. Contact a clinician, and seek care right away if you feel very unwell or have ketones in your urine.