A1C — The Number That Matters Most

What Exactly Does A1C Measure?

The hemoglobin A1C test — often just called "A1C" or "HbA1c" — measures the percentage of hemoglobin proteins in your red blood cells that have glucose attached to them. Because red blood cells live for approximately 2–3 months, the A1C reflects your average blood glucose over that entire period, providing a longer-term picture than any single glucose reading.

Think of it this way: if your daily glucose readings are individual snapshots, your A1C is the album. It tells a story that a single fasting glucose test cannot.

How to Read Your A1C Results

The ADA recommends that most non-pregnant adults with diabetes target an A1C below 7%. For some individuals — including older adults, those with frequent hypoglycemia, or those with limited life expectancy — a less stringent target (such as below 8%) may be appropriate. Conversely, younger, healthier individuals with a long life expectancy may benefit from targets below 6.5%. Your target should be set with your healthcare provider.

The Limitations of A1C

A1C is a powerful tool, but it has important limitations that are frequently overlooked:

• It hides variability: Two people can have an identical A1C of 7% — one because they consistently maintain glucose around 150 mg/dL, another because they alternate between severe lows and severe highs. The second pattern is associated with more harm despite the same average.
• Conditions affecting red blood cell lifespan: Anemia (especially iron-deficiency or hemolytic anemia) can falsely lower A1C. Conditions like sickle cell trait may produce unreliable A1C readings.
• Ethnicity-related differences: Several studies have found that A1C may be systematically higher in some ethnic groups (particularly African Americans) compared to white Americans at equivalent average glucose levels, which has implications for interpretation.
• It doesn't capture time-in-range: Continuous glucose monitoring (CGM) and its "time-in-range" metric (the percentage of time glucose stays between 70–180 mg/dL) adds meaningful information that A1C cannot provide.

How Often Should A1C Be Tested?

General guidance from the American Diabetes Association:

• For people meeting treatment goals with stable glycemia: At least twice per year
• For people whose therapy has changed, or who are not meeting goals: Every 3 months
• At initial diagnosis: Immediately, then regularly to track response to treatment

What Can Move Your A1C?

Because A1C reflects a 3-month average, meaningful changes generally take at least 3 months to show up in test results. Interventions with consistent evidence for A1C improvement include:

• Dietary changes: Reducing refined carbohydrates and added sugars lowers post-meal glucose spikes, which can meaningfully improve A1C over time. The Mediterranean and low-carbohydrate diets both have evidence for A1C reduction in type 2 diabetes.
• Regular physical activity: Exercise improves insulin sensitivity and reduces post-meal glucose. Meta-analyses consistently show that aerobic exercise, resistance training, and combined programs all reduce A1C by approximately 0.5–0.7% on average in people with type 2 diabetes.
• Weight management: In people with excess body weight, even modest weight loss of 5–10% is associated with significant A1C improvement.
• Sleep optimization: Chronic poor sleep is associated with higher A1C. Addressing sleep disorders, particularly sleep apnea, has shown A1C benefits in several studies.
• Stress reduction: Chronic stress elevates cortisol, which promotes insulin resistance and hepatic glucose production. Behavioral interventions targeting stress have shown modest but measurable A1C benefits.
• Medication: Various classes of diabetes medications reduce A1C through different mechanisms. Work with your prescriber to find the right combination for your situation.

Beyond A1C: Time-in-Range

As continuous glucose monitors (CGMs) become more accessible, the metric of time-in-range (TIR) is gaining clinical attention as a complement to A1C. TIR measures the percentage of time your glucose stays in a target range (usually 70–180 mg/dL). Research suggests that a TIR above 70% correlates with lower rates of diabetes complications. See our CGM guide to learn more about tracking these metrics.