Understanding HbA1c Testing

The HbA1c test, usually called the A1C test in the United States, is one of the most important measurements in diabetes care. Unlike a finger-prick reading, which reflects a single moment, HbA1c shows how blood glucose has been running over the previous two to three months.

What HbA1c measures

Hemoglobin is the protein in red blood cells that carries oxygen. Glucose in the blood attaches to it; the more glucose in the blood, the more hemoglobin becomes “glycated”. Because red blood cells live for around three months, the proportion of glycated hemoglobin reflects average glucose over that period.

How the test is done

A blood sample is taken from a vein in the arm or from a finger prick and analyzed in a laboratory or on a clinic device. You do not need to fast.

Understanding the result

In the United States, results are given as a percentage. The CDC lists these ranges:

• Normal: below 5.7%
• Prediabetes: 5.7% to 6.4%
• Diabetes: 6.5% or above

A diagnosis is normally confirmed with a second test.

Percent and mmol/mol

Some countries, including the United Kingdom, report HbA1c in millimoles per mole (mmol/mol). For reference, 6.5% equals 48 mmol/mol and 7% equals 53 mmol/mol.

Estimated average glucose

An HbA1c result can be converted into an estimated average glucose (eAG), in the same units as your meter:

• 6% is about 126 mg/dL
• 7% is about 154 mg/dL
• 8% is about 183 mg/dL
• 9% is about 212 mg/dL

Targets for people with diabetes

The goal for many adults with diabetes is 7% or less. Your healthcare team may set a lower or higher target depending on your age, other health conditions, how long you have had diabetes and your risk of low blood sugar. Diabetes UK notes that the ideal level for most people with diabetes is 48 mmol/mol (6.5%) or below, while emphasizing that targets are individual.

How often to test

• At least twice a year if you are meeting your treatment goals.
• Every three months if your treatment has changed or you are not meeting your goals.

HbA1c and daily monitoring

The two measures answer different questions.

• Home readings show highs and lows through the day and guide immediate decisions.
• HbA1c summarizes overall control and predicts the risk of long-term complications.

Two people can have the same HbA1c with very different daily patterns: one stable, the other swinging between highs and lows. That is why daily readings, or CGM data, still matter.

What can affect accuracy

The NIDDK lists conditions that can make HbA1c results falsely high or low:

• anemia and other blood disorders;
• hemoglobin variants, such as sickle cell trait;
• kidney failure or liver disease;
• recent blood loss or transfusion;
• pregnancy;
• certain medicines.

If one of these applies, your doctor may use other tests to assess glucose control.

Home A1C kits

Kits for home use are available. They can be convenient between appointments, but results may differ from a laboratory test, and they should not be used to diagnose diabetes. Discuss any home result with your doctor.

Lowering HbA1c

HbA1c falls when average glucose falls. That generally means following your meal plan, staying active, taking medicines as prescribed and monitoring so that adjustments can be made. Because the test reflects months, changes take time to show; a follow-up test is usually done about three months after a change in treatment.

Questions to ask your doctor

• What is my HbA1c, and what is my target?
• How does it compare with my last result?
• Do any of my conditions affect the accuracy of the test?
• When should I be tested again?

The bottom line

HbA1c is the long view of your blood glucose. Know your number, your target and when your next test is due, and use daily monitoring to understand the story behind it. Find home testing supplies in our diabetes care department.

Sources

1. Centers for Disease Control and Prevention. A1C Test for Diabetes and Prediabetes.
2. National Institute of Diabetes and Digestive and Kidney Diseases. The A1C Test & Diabetes.
3. Diabetes UK. What is HbA1c?.
4. American Diabetes Association. Understanding A1C.