An A1C test measures your average blood sugar level over the past two to three months. It works by measuring how much glucose has attached to hemoglobin, the oxygen-carrying protein inside your red blood cells. The result comes back as a percentage: the higher the percentage, the higher your average blood sugar has been. It’s one of the primary tools used to diagnose diabetes and prediabetes, and to monitor how well diabetes management is working over time.
How the A1C Test Works
Glucose in your bloodstream naturally sticks to hemoglobin. The more glucose circulating in your blood, the more hemoglobin gets coated. Once glucose attaches, it stays there for the life of the red blood cell, which is about three months. That’s why the test reflects a two-to-three-month average rather than a single moment in time.
This makes the A1C fundamentally different from a finger-stick glucose reading or a fasting blood sugar test, both of which capture your blood sugar right now. A single high-carb meal, a stressful morning, or a skipped medication can swing those numbers dramatically. The A1C smooths all of that out into one number that reflects the bigger picture of your blood sugar control.
What the Numbers Mean
A1C results fall into three general ranges:
- Below 5.7%: Normal blood sugar levels.
- 5.7% to 6.4%: Prediabetes. Blood sugar is higher than normal but not yet in the diabetes range.
- 6.5% or higher: Diabetes. A second test is usually done to confirm the diagnosis.
For people already managing diabetes, the A1C also translates into an estimated average glucose (eAG), which can make the number feel more concrete. An A1C of 7% corresponds to an average blood sugar of roughly 154 mg/dL. An A1C of 9% corresponds to about 212 mg/dL. Many treatment plans aim for an A1C below 7%, though your target may differ depending on your age, health, and risk of low blood sugar episodes.
No Fasting Required
You don’t need to fast before an A1C test. Because it measures glucose that has accumulated on your red blood cells over months, what you ate last night or this morning won’t change the result. That said, your doctor may order other blood tests at the same visit, like a cholesterol panel, that do require fasting. If you’re unsure, ask ahead of time whether you need to skip breakfast.
A short-term change in diet, like eating well for a week before your appointment, also won’t meaningfully shift your A1C. The number reflects the full two-to-three-month window, so it’s a reliable indicator of your day-to-day blood sugar patterns, not just your recent behavior.
How Often You Should Get Tested
Testing frequency depends on where you are with your blood sugar management. If you have diabetes and you’re meeting your treatment goals, testing every six months is typical. If your treatment recently changed or you’re having trouble staying in your target range, every three months is more appropriate. For people with prediabetes or risk factors for diabetes, your doctor may order the test annually or as part of routine bloodwork.
A1C vs. Daily Blood Sugar Monitoring
These two tools answer different questions. The A1C gives you the long view: how well your overall management strategy is working across weeks and months. Daily blood sugar checks, whether by finger stick or a continuous glucose monitor, give you real-time information you can act on immediately, like adjusting an insulin dose before a meal or catching a low before it becomes dangerous.
Neither replaces the other. Think of daily monitoring as the day-to-day steering, and the A1C as a quarterly report card. Someone could have a decent A1C but still experience dangerous highs and lows that average out to a normal-looking number. That’s why daily monitoring matters even when A1C results look good.
Conditions That Can Skew Results
The A1C test assumes your red blood cells have a normal lifespan of about three months. When that assumption doesn’t hold, results can be misleading.
Anything that shortens the life of your red blood cells, such as hemolytic anemia or recovery from significant blood loss, will make your A1C appear falsely low. The glucose-coated cells are destroyed before they accumulate a full three months’ worth of sugar. On the other hand, iron deficiency anemia tends to push A1C readings falsely high. Iron replacement therapy has been shown to lower A1C in both diabetic and non-diabetic individuals, suggesting that some elevated readings may partly reflect an iron problem rather than a sugar problem.
Genetic hemoglobin variants, including sickle cell trait and hemoglobin C trait, can also interfere with accuracy depending on the laboratory method used. People with sickle cell disease face compounded challenges because anemia, faster red blood cell turnover, and frequent transfusions all distort the result. Chronic kidney disease, particularly for people on dialysis, can cause A1C to underestimate true blood sugar levels. In these situations, doctors may use alternative markers like fructosamine or glycated albumin, which measure sugar attachment to different proteins and reflect a shorter time window.
Certain medications can also affect results. Opioids and some HIV medications have been identified as drugs that can falsely raise or lower A1C readings. Pregnancy, especially in the third trimester, can push A1C slightly higher in people without diabetes due to pregnancy-related iron deficiency. If you have any of these conditions, your doctor may interpret your A1C alongside other glucose measures rather than relying on it alone.
How Results Are Reported
In the United States, A1C is reported as a percentage, a system tied to the landmark clinical trials that established the link between A1C levels and diabetes complications. Many other countries report results in mmol/mol, a unit developed by the International Federation of Clinical Chemistry. The two systems are mathematically linked, so they convey the same information in different formats. Your lab report may include both units along with your estimated average glucose in mg/dL or mmol/L, depending on where you live.
Laboratories that run A1C tests are certified through a standardization program that traces back to those original clinical trials, ensuring that a result of 7% means the same thing regardless of which lab processes your blood. This consistency is what makes A1C a reliable benchmark for tracking your progress over time, even if you switch doctors or move to a different city.