You can test your A1C through a standard blood draw at a lab, a quick fingerstick at your doctor’s office, or an at-home test kit you buy over the counter. No fasting or special preparation is required for any of these methods. The 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 the past two to three months.
What the A1C Test Actually Measures
Glucose in your bloodstream naturally sticks to hemoglobin, the protein inside red blood cells that carries oxygen. This binding happens continuously and irreversibly: once glucose attaches, it stays for the life of that red blood cell, which is about 120 days. The A1C test counts what percentage of your hemoglobin has glucose stuck to it. A higher percentage means your blood sugar has been running higher over recent months.
The reading is weighted toward the recent past. About half of the glycation reflected in your result happened in the last 30 days. Another 40% comes from days 31 through 90, and only about 10% from beyond 90 days. So while the test captures a two-to-three-month window, your most recent weeks influence the number more heavily than anything that happened three months ago.
The Three Ways to Get Tested
Lab Blood Draw
This is the gold standard. A technician draws blood from a vein in your arm, and the sample goes to a lab with advanced equipment designed to minimize error. Lab results are precise enough to diagnose prediabetes or diabetes on their own. If your doctor orders an A1C, this is typically what happens, and results usually come back within a day or two.
Point-of-Care Testing at a Clinic
Many doctor’s offices and pharmacies use a fingerstick device that analyzes a small drop of blood on-site. You get results in minutes, often during the same appointment. These devices are calibrated to lab standards and are reliable enough for routine monitoring, though your provider may confirm an unexpected result with a full lab draw.
At-Home Test Kits
Over-the-counter A1C kits are available at most pharmacies and online. You prick your finger, place a blood drop on a test card, and either read the result from the device or mail the card to a lab. Some are FDA-cleared and have undergone validation studies, but their accuracy depends heavily on technique. Improper sample collection, storage conditions, and handling can all throw off results. Because of this variability, at-home kits cannot be used to diagnose diabetes. They’re best suited for tracking trends between doctor visits, not for making treatment decisions on your own.
No Fasting Required
Unlike a fasting blood glucose test, the A1C requires zero preparation. You can eat, drink, exercise, and take your medications as usual before the test. This is one of the reasons it became a preferred diagnostic tool: it captures your average blood sugar regardless of what you ate for breakfast that morning.
What Your Results Mean
The CDC uses these ranges for diagnosis:
- Below 5.7%: Normal
- 5.7% to 6.4%: Prediabetes
- 6.5% or above: Diabetes
A single result at or above 6.5% is typically confirmed with a second test before a formal diagnosis. If you’re already managing diabetes, your target A1C will depend on your individual situation, but many people aim to stay below 7%.
How Often to Test
Clinical guidelines recommend testing every six months if your blood sugar is well controlled and every three months if it’s not, or if you’ve recently changed your treatment plan. Interestingly, research has found that testing every three months doesn’t necessarily improve blood sugar control in people who are already stable, so twice a year is sufficient for many people. If you’re using at-home kits to check between appointments, spacing them about three months apart lines up well with how the test works, since it takes that long for your red blood cell population to fully turn over.
When A1C Results Can Be Misleading
The test assumes your red blood cells live a normal lifespan. Anything that changes how long your red blood cells survive will skew the number.
Conditions that shorten red blood cell life, like hemolytic anemia, significant blood loss, or sickle cell disease, will push your A1C falsely low. Your red blood cells aren’t around long enough to accumulate the usual amount of glucose, so the test underestimates your true average blood sugar. People with sickle cell trait or other hemoglobin variants need to interpret their results with particular caution.
On the other hand, iron deficiency anemia can push results falsely high. This also applies to late pregnancy, where iron deficiency is common. Even in nondiabetic pregnant individuals, A1C readings can appear elevated due to changes in iron status rather than actual blood sugar problems. Kidney failure can also interfere, as chemically modified forms of hemoglobin in patients on dialysis affect the accuracy of certain testing methods.
Alternatives When A1C Isn’t Reliable
If you have a condition that makes A1C unreliable, your doctor may use a fructosamine test instead. This test works on a similar principle but measures glucose attached to proteins in your blood (rather than hemoglobin), reflecting your average blood sugar over just two to three weeks instead of two to three months. The shorter window makes it useful in two situations: when A1C can’t be trusted due to red blood cell abnormalities, and when you’ve recently changed your treatment plan and want to see whether the change is working without waiting three months for an A1C to catch up.
Continuous glucose monitors offer another option. They track blood sugar in real time and can calculate a “glucose management indicator” that estimates your A1C from actual glucose data, bypassing hemoglobin entirely.