The HbA1c blood test measures the percentage of your hemoglobin (the oxygen-carrying protein in red blood cells) that has glucose attached to it. A normal result is below 5.7%, a result between 5.7% and 6.4% indicates prediabetes, and 6.5% or higher means diabetes. Unlike a finger-stick glucose check that captures a single moment, the A1c reflects your average blood sugar over the past two to three months.
How Glucose Attaches to Hemoglobin
Glucose circulates in your bloodstream all the time. As it does, some of it spontaneously sticks to the hemoglobin inside your red blood cells. This isn’t an enzyme-driven process your body controls. It happens passively: the more glucose floating around, the more that latches on. Once glucose binds to hemoglobin, it stays there for the life of that red blood cell.
Red blood cells regenerate roughly every three months. That turnover is the reason an A1c result represents a two-to-three-month average rather than a snapshot. Older red blood cells have had more time to accumulate glucose, while younger ones have had less. The test captures the mix of old and new cells in your blood at the time of the draw, producing a weighted average that leans more heavily toward the most recent four to six weeks.
What the Numbers Mean
Results are reported as a percentage. Here’s how the American Diabetes Association classifies them:
- Below 5.7%: Normal blood sugar regulation.
- 5.7% to 6.4%: Prediabetes. Blood sugar has been higher than ideal, and the risk of developing type 2 diabetes is elevated.
- 6.5% or higher: Diabetes range. A second confirmatory test is typically needed unless symptoms are already present.
For people already diagnosed with diabetes, most treatment plans aim to keep A1c below 7%, though your target may be individualized. Each percentage point above normal corresponds to meaningfully higher risk of complications affecting the eyes, kidneys, nerves, and cardiovascular system.
Converting A1c to Average Blood Sugar
A percentage can feel abstract, so labs often include an estimated average glucose (eAG) alongside your A1c. The conversion formula is straightforward: multiply your A1c by 28.7, then subtract 46.7. The result is your estimated average blood sugar in mg/dL.
For example, an A1c of 6.0% translates to an eAG of about 126 mg/dL. An A1c of 7.0% works out to roughly 154 mg/dL. This gives you a number you can compare directly to the readings on a home glucose meter, making it easier to see how your day-to-day numbers connect to the bigger picture.
No Fasting Required
One practical advantage of the A1c test is that it doesn’t require fasting. You can eat and drink normally before your blood draw. That’s because the result reflects months of glucose exposure, not what you ate for breakfast. A single meal, a stressful morning, or a particularly active day won’t move the number. This makes it convenient to schedule alongside other routine lab work regardless of the time of day.
When the Test Can Be Misleading
Because the A1c depends on both your glucose levels and the normal lifespan of your red blood cells, anything that disrupts red blood cell turnover can skew results. Conditions like iron-deficiency anemia, sickle cell disease, significant kidney disease, or liver failure can shorten or lengthen the time red blood cells survive in circulation. Shorter-lived cells have less time to accumulate glucose, which can push the A1c falsely low. Longer-lived cells do the opposite.
Certain inherited hemoglobin variants can also interfere with specific lab methods used to measure A1c, producing inaccurate readings. This doesn’t mean the test is useless for people with these conditions, but your doctor may choose a different testing method or rely on alternative markers like fructosamine, which reflects a shorter window of blood sugar control.
Pregnancy is another situation where A1c interpretation gets complicated. Red blood cell production speeds up during pregnancy, and iron status shifts, both of which can affect accuracy. Gestational diabetes screening typically relies on oral glucose tolerance testing instead.
How It Differs From Other Glucose Tests
A fasting glucose test measures your blood sugar after an overnight fast, giving a single data point. An oral glucose tolerance test measures how quickly your body clears a standard dose of sugar. Both are useful, but both capture only what’s happening in that moment. Someone with well-controlled fasting glucose could still have significant spikes after meals that only show up in a higher A1c.
The A1c fills a different role: it’s the long view. It smooths out the daily highs and lows into one number that reflects overall glucose control. That’s why it’s used both for initial diagnosis and for ongoing monitoring in people managing diabetes. Most people with diabetes get the test two to four times per year, depending on how stable their blood sugar has been.
What Happens During the Test
The A1c is a standard blood draw, usually from a vein in your arm. Some clinics use a point-of-care device that requires only a finger prick and returns results in minutes. In the U.S., results are reported as a percentage following the NGSP standard. Some countries report in mmol/mol using the IFCC standard, so if you’re comparing results from different healthcare systems, the units may look different even though they measure the same thing. Your lab report will specify which unit is being used.