What is the difference between HbA1c in % and mmol/mol?

They are two units for the same test. US labs usually report a percentage and UK labs mmol/mol. 6.5% corresponds to 48 mmol/mol.

Do I need to fast for an HbA1c test?

Lab Tests Online UK says no preparation is needed. Follow any instructions you were given, especially if other tests are being done.

What can make an HbA1c result inaccurate?

Conditions that affect red blood cells, such as anemia or hemoglobin variants, heavy bleeding, recent transfusion and pregnancy, among others.

HBA1C

HbA1c result explained

HbA1c (A1C) is a blood test that reflects your average blood sugar over the past two to three months, by measuring how much of the hemoglobin in your red blood cells has glucose attached. It is used to screen for and diagnose type 2 diabetes and prediabetes, and to monitor diabetes.

Why it’s confusing

It is reported two ways. US labs use a percentage; UK labs use millimoles per mole (mmol/mol). The same result looks completely different — 6.5% and 48 mmol/mol are the diagnostic threshold in each unit.

It also behaves differently from a finger-prick glucose reading. A glucose test shows your level at that moment; HbA1c shows an average, because glucose stays attached to hemoglobin for the life of the red cell — about three months.

How to read it

In percentages, MedlinePlus lists the ranges commonly used to diagnose diabetes or prediabetes: normal below 5.7%, prediabetes 5.7% to 6.4%, and diabetes 6.5% or higher.

In mmol/mol, Lab Tests Online UK gives the WHO guideline: below 42 not diabetes, 42 to 47 prediabetes, and 48 or over indicative of diabetes — with repeat testing to confirm if there are no symptoms.

Doctors often use more than one test to diagnose diabetes. If you already have diabetes, your target is set with your care team and may differ from these figures.

Some things can make HbA1c less accurate, including anemia, some hemoglobin variants, heavy bleeding, recent transfusion and pregnancy.

What it does not mean

That one result confirms a diagnosis. A repeat test or a different test is often used.
That the diagnostic ranges are your personal target if you have diabetes. Your care team sets that.
That it reflects what you ate yesterday. It is a two-to-three-month average.

Where you’ll see it

Diabetes screening and monitoring results.
NHS Health Check and routine blood test reports, often as “HbA1c” or “glycated hemoglobin”.

Questions worth asking

Ask your doctor

What does my HbA1c mean for me?
Do I need a repeat test or another type of test?
Could anything have affected the result?

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Common questions

What is the difference between HbA1c in % and mmol/mol?: They are two units for the same test. US labs usually report a percentage and UK labs mmol/mol. 6.5% corresponds to 48 mmol/mol.
Do I need to fast for an HbA1c test?: Lab Tests Online UK says no preparation is needed. Follow any instructions you were given, especially if other tests are being done.
What can make an HbA1c result inaccurate?: Conditions that affect red blood cells, such as anemia or hemoglobin variants, heavy bleeding, recent transfusion and pregnancy, among others.

Sources

Reading a scan report rather than a blood test or letter? Radily’s radiology report phrase guides explain the wording radiologists use.