An A1C test measures the percentage of your hemoglobin (a protein in red blood cells) that has glucose attached to it, giving you a picture of your average blood sugar over roughly the past two to three months. Unlike a finger-stick glucose reading that captures a single moment, A1C reflects the bigger trend. You can get it done at a lab, at your doctor’s office with a point-of-care device, or even with an at-home kit, and no fasting is required.
What the Test Actually Measures
Glucose in your bloodstream naturally sticks to hemoglobin inside red blood cells through a process called glycation. Because red blood cells can’t regulate how much glucose they absorb, the amount of sugar that attaches to hemoglobin directly mirrors how much sugar has been floating in your blood. The higher your blood sugar has been, the more glycated hemoglobin you’ll have.
Red blood cells live for a fixed period, roughly 120 days, before your body replaces them. That lifespan is why A1C captures an average over two to three months rather than a single day. At any given time your blood contains red blood cells of all different ages, from brand new to nearly expired, so the test blends recent weeks and older weeks together. In practice, the most recent 30 days influence the result more heavily than the first 30 days of that window, because younger red blood cells make up a larger share of the total.
How A1C Is Measured in a Lab
Laboratories use several techniques to separate glycated hemoglobin from normal hemoglobin and calculate the percentage. The two most common approaches are chromatography, which physically separates hemoglobin types based on their chemical properties, and immunoassay, which uses antibodies that bind specifically to glycated hemoglobin. Both are accurate but require specialized instruments.
Point-of-care analyzers in a doctor’s office work on similar principles but use just a drop of blood and return results in minutes rather than days. These are convenient for making treatment decisions during a single visit.
At-Home A1C Kits
Over-the-counter A1C tests let you collect a small blood sample at home and either read the result on a test device or mail the sample to a lab. Two products, Home Access and A1cNow+, are cleared by the FDA for home use. Their accuracy varies considerably, though. In a study of 219 people with diabetes, researchers compared home kit results against a standard lab draw. The accuracy benchmark: at least 90% of samples should fall within 5% of the lab value.
Home Access came closest, with 82% of its samples meeting that benchmark. A1cNow+ hit the mark in only 46% of samples. A third test, CoreMedica, managed just 29%. So while home kits can be useful for tracking trends between doctor visits, they aren’t a substitute for a lab-quality result when a precise number matters for diagnosis or treatment changes.
Interpreting Your Results
The American Diabetes Association uses these cutoffs:
- Below 5.7%: Normal range
- 5.7% to 6.4%: Prediabetes
- 6.5% or higher: Diabetes range
You can translate any A1C percentage into an estimated average glucose (eAG) using a simple formula: multiply the A1C by 28.7, then subtract 46.7. The result is in mg/dL. For example, an A1C of 7% corresponds to an eAG of about 154 mg/dL. This conversion can make the number feel more concrete if you’re used to seeing daily glucose readings on a meter or continuous monitor.
How Often to Get Tested
The CDC recommends testing every six months if your blood sugar is well controlled and your treatment plan is stable. If your treatment has recently changed or you’re struggling to meet your blood sugar goals, every three months is more appropriate. Testing more frequently than every three months rarely adds useful information, since the result mostly reflects the same overlapping window of red blood cell life.
Conditions That Skew A1C Results
Because A1C depends on hemoglobin and red blood cell lifespan, anything that alters either one can throw off the number, sometimes significantly.
Hemoglobin variants are the most well-known interference. People with sickle cell trait (hemoglobin S), for instance, can get falsely high or low readings depending on the lab method used. Hemoglobin E trait can artificially lower results when the lab’s instrument doesn’t detect the variant. Elevated fetal hemoglobin, which occurs in certain inherited blood disorders, also skews results across most testing methods, including immunoassay and some chromatography techniques. The National Glycohemoglobin Standardization Program considers any distortion greater than 6% at clinically important A1C levels to be significant enough to mislead treatment decisions.
Conditions that shorten red blood cell lifespan, such as hemolytic anemia or significant blood loss, tend to pull A1C artificially low because cells don’t live long enough to accumulate the expected amount of glucose. Conditions that lengthen red blood cell lifespan, like iron deficiency anemia, can push A1C artificially high for the opposite reason. If your A1C doesn’t match what your daily glucose readings suggest, one of these factors may be at play, and your doctor may rely on alternative markers like fructosamine instead.
Why A1C Is Unreliable During Pregnancy
A1C is not recommended for diagnosing gestational diabetes. Pregnancy changes red blood cell turnover in ways that make the test underestimate actual blood sugar. Pregnant women produce new red blood cells at a faster rate and experience hemodilution, meaning their blood volume increases and dilutes hemoglobin concentration. The result is that A1C reads lower than it should.
Research from Massachusetts General Hospital found that by the late second trimester, actual average glucose (measured by an oral glucose tolerance test) ran about 13 mg/dL higher than what A1C would have predicted. Women with lower hemoglobin levels showed an even bigger gap. This means a reassuring-looking A1C could mask blood sugar levels that actually need treatment. For this reason, the oral glucose tolerance test remains the standard for diagnosing gestational diabetes.
Preparation and Practical Tips
No fasting is required before an A1C draw, and time of day doesn’t matter. A recent meal won’t change the result because the test measures glucose exposure accumulated over months, not the sugar in your blood right now. That said, your doctor may bundle A1C with other bloodwork, like a cholesterol panel, that does require fasting. If you’re unsure, ask when scheduling whether you need to skip breakfast.
If you have a known hemoglobin variant, let the lab know before your blood is drawn. Many labs can choose a testing method that isn’t affected by your specific variant, ensuring you get an accurate number. If you’ve recently had a blood transfusion or significant blood loss, the result may not reflect your true average, and retesting after a few months is more informative.