The A1C test, also called the hemoglobin A1C or HbA1c test, is the number most doctors use to diagnose prediabetes and type 2 diabetes and to check how well treatment is working. This article explains what the test measures, what the ranges mean and why your result can sometimes differ from your home readings.

What the test measures

Glucose in your blood sticks to hemoglobin, the protein in red blood cells that carries oxygen. The higher your blood sugar has been, the more of your hemoglobin ends up with glucose attached. The A1C result is the percentage of hemoglobin that has glucose attached to it.

Red blood cells live for about three months, so the A1C reflects your average blood sugar over roughly the past two to three months. A single high reading after a large meal will barely move it, while weeks of higher numbers will.

The ranges

The American Diabetes Association uses these cut-offs for diagnosis:

• Below 5.7%: normal.
• 5.7% to 6.4%: prediabetes.
• 6.5% or higher: diabetes, usually confirmed with a second test.

Two other tests use matching ranges. A fasting blood sugar of 100 to 125 mg/dL counts as prediabetes and 126 mg/dL or higher as diabetes. On a two-hour oral glucose tolerance test, 140 to 199 mg/dL counts as prediabetes and 200 mg/dL or higher as diabetes.

Turning A1C into an average reading

Because most people know their meter readings better than percentages, labs often convert the A1C into an estimated average glucose. An A1C of 6% corresponds to an average of about 126 mg/dL, 7% to about 154 mg/dL, 8% to about 183 mg/dL and 9% to about 212 mg/dL. Each full point on the A1C scale is roughly 29 mg/dL of average blood sugar.

What target to aim for

For many adults with diabetes, the American Diabetes Association suggests an A1C below 7%. Your own target may be higher or lower. Doctors often set a looser goal for older adults, for people who have had serious low blood sugar episodes and for people with other major health conditions, and a tighter goal for some people early in the disease when it can be reached safely.

Why your A1C and your meter can disagree

Some conditions change how long red blood cells live or how glucose attaches to hemoglobin, which can make the A1C read higher or lower than your true average. These include iron deficiency and other types of anemia, recent heavy blood loss or a blood transfusion, advanced kidney disease, pregnancy and certain inherited hemoglobin variants such as sickle cell trait.

The timing of home checks also matters. If you mostly test first thing in the morning, you may never see the higher numbers after meals that the A1C includes. A continuous glucose monitor or a week of checks two hours after meals can show where the gap comes from.

How often to test

The American Diabetes Association recommends an A1C test at least twice a year for people who are meeting their goals and about every three months for people whose treatment has recently changed or who are above their target. People with prediabetes are usually retested once a year.

Questions to ask about your result

• What A1C target is right for me, and why?
• Has my number gone up, down or stayed the same since my last test?
• Do I have any condition, such as anemia, that could affect the accuracy of the test?
• Should I check my blood sugar at home, and at what times of day?