An A1C test measures the percentage of your red blood cells that have glucose attached to them, giving a snapshot of your average blood sugar over roughly two to three months. It can be done with a standard blood draw at a lab or with an at-home fingerstick kit. No fasting is required beforehand, though your doctor may order other bloodwork at the same time that does require fasting.
What the Test Actually Measures
Glucose in your bloodstream naturally sticks to hemoglobin, the protein inside red blood cells that carries oxygen. The more glucose circulating in your blood over time, the more hemoglobin gets coated. Because red blood cells live about 120 days before your body replaces them, an A1C reading reflects your average blood sugar across that entire lifespan rather than a single moment in time. That’s what makes it useful for tracking long-term blood sugar control in a way that a daily finger-prick glucose check cannot.
How It’s Done at a Lab or Clinic
A lab A1C requires about a teaspoon of blood drawn from a vein, typically in your arm. The sample goes into a tube and is processed as whole blood. Results usually come back within a day or two, depending on the lab. The draw itself takes a few minutes, and because fasting isn’t needed, you can have it done at any time of day.
How to Test A1C at Home
Home A1C kits use a small fingerstick blood sample instead of a full blood draw. The most widely available consumer kit, the A1CNow Self Check, comes with a lancet to prick your finger, a blood collector, a shaker pouch for mixing the sample, and a handheld analyzer that displays your result. The entire process takes less than 10 minutes from start to finish.
A few details matter for getting an accurate home reading. Once you collect your blood sample, you need to place it in the analyzer within two minutes. Don’t move or pick up the analyzer while it’s processing. When your result appears on screen, write it down immediately. The device does not save the reading after 15 minutes, and it will shut off on its own.
Home kits are convenient for tracking trends between doctor visits, but lab-based tests are considered more precise. If a home result seems off or if you’re using the number to make treatment decisions, a lab draw is the better option.
Understanding Your Results
A1C is reported as a percentage. The standard diagnostic thresholds are:
- Below 5.7%: Normal
- 5.7% to 6.4%: Prediabetes
- 6.5% or above: Diabetes
You can also convert an A1C percentage into an estimated average glucose (eAG) to get a number that looks more like what you’d see on a daily glucose meter. The formula is: eAG in mg/dL = (28.7 × A1C) − 46.7. So an A1C of 7% translates to an average blood sugar of about 154 mg/dL, and an A1C of 6% works out to roughly 126 mg/dL. Many lab reports now include this conversion automatically.
How Often to Test
If you’re managing diabetes and haven’t yet reached stable blood sugar targets, testing every three months gives enough time for changes in diet, exercise, or medication to show up in the results. Once your levels are stable, every six months is typically sufficient. More frequent testing may be warranted for children with type 1 diabetes, people planning a pregnancy, or anyone who has recently made major changes to their lifestyle or medications.
Because the test reflects a two-to-three-month window, retesting sooner than about 60 days won’t give you meaningfully new information. Labs sometimes enforce a minimum interval of 60 to 86 days between tests for this reason.
When A1C Results Can Be Misleading
Several health conditions can push your A1C reading higher or lower than your actual blood sugar control would suggest. Knowing these is important because an inaccurate number could lead to the wrong treatment plan.
Conditions That Falsely Raise A1C
Iron deficiency anemia is one of the most common culprits. When you’re low on iron, your red blood cells live longer than usual, giving glucose more time to accumulate on hemoglobin. The same applies to vitamin B12 and folate deficiency anemias. Chronic alcohol use, chronic opioid use, and lead poisoning have also been linked to falsely elevated readings.
Conditions That Falsely Lower A1C
Anything that shortens the lifespan of your red blood cells will bring the number down artificially. Acute or chronic blood loss, hemolytic anemia (where red blood cells break down faster than normal), and an enlarged spleen all fall into this category. End-stage kidney disease also tends to produce falsely low readings.
Pregnancy is a notable case. During pregnancy, red blood cells turn over faster, living closer to 90 days instead of 120. A1C values tend to read falsely low through the second trimester and may rise during the third. This is one reason doctors often rely on other glucose tests, like the oral glucose tolerance test, for diagnosing gestational diabetes.
Hemoglobin Variants
People who carry hemoglobin variants, including sickle cell trait and other inherited hemoglobin types, can see results skewed in either direction depending on the specific variant and the lab method used. If you know you carry a hemoglobin variant, mention it to your doctor so they can choose a testing method that accounts for it or rely on alternative measures of blood sugar control.