Hemoglobin A1c (HbA1c)

What is Hemoglobin A1c (HbA1c)?

HbA1c measures the share of your hemoglobin that's coated in glucose, reflecting your average blood sugar over roughly the past three months.

Glucose in your blood attaches to hemoglobin, the oxygen-carrying protein inside red blood cells, through a slow chemical process called glycation. It isn't an enzyme reaction and it isn't reversible: once glucose binds, it stays bound for the life of that red blood cell. The more glucose that's circulated through your bloodstream, the more of your hemoglobin ends up glycated. Red blood cells live about three to four months, and that lifespan is what sets the HbA1c test's window. But the test doesn't weigh every day equally. Older red blood cells, carrying older glycation, die off and get replaced continuously, so blood drawn today reflects a blend that's skewed toward whatever happened most recently, with glucose levels from the past 30 days carrying more influence than the 60 to 90 days before that, according to NIDDK. Results are reported as a percentage of total hemoglobin that's glycated. Outside the United States, labs often report the same measurement in mmol/mol under the IFCC standard, so a result of 6.5% corresponds to roughly 48 mmol/mol. Because it's an average built from a slow biological process, HbA1c doesn't require fasting and can be drawn at any time of day, one reason clinics use it alongside, not instead of, a same-day glucose reading.

Typical reference context

These are the diagnostic cutoffs published in the American Diabetes Association's Standards of Care in Diabetes—2026 (Diabetes Care, supplement to the January 2026 issue, released December 8, 2025), consistent with NIDDK. They're generic clinical thresholds set by these organizations, not an interpretation of what any individual's number means.

What changed recently

The 2026 ADA Standards of Care carried the A1C diagnostic cutoffs forward unchanged, so the genuinely new development for this marker is on the measurement side, not the diagnostic side. A study published in Diabetologia on April 24, 2026 (Lenters-Westra et al.) analyzed 26,647 paired average-glucose and HbA1c readings from continuous glucose monitors and found that the standard Glucose Management Indicator, a CGM-based estimate meant to approximate lab HbA1c, overestimates HbA1c by about 4.4 mmol/mol at low levels and underestimates it by about 3.3 mmol/mol at high levels. The researchers proposed an updated calculation that folds in glucose transport kinetics and red blood cell lifespan, cutting that bias to roughly 1.1 mmol/mol or less at the extremes. For anyone who wears a CGM and also gets lab HbA1c drawn, this explains why the two numbers can disagree, especially outside the middle of the range.

Tests often checked alongside this

• Fasting glucose — a same-day snapshot that shows whether a recent reading lines up with the three-month average HbA1c reflects.
• Fructosamine — reflects glycation over roughly two to three weeks, useful when a shortened red blood cell lifespan is throwing off HbA1c itself.
• Complete blood count — flags anemia or other red-blood-cell issues that can distort an HbA1c reading independent of actual glucose.
• Kidney function tests (eGFR, creatinine) — chronic kidney disease changes red blood cell survival and is a documented source of HbA1c inaccuracy.
• CGM-derived Glucose Management Indicator — a device-based estimate that can diverge from lab HbA1c at the high and low ends.

What can affect this marker

• Anemia and recent blood loss: conditions that shorten red blood cell lifespan change how long glucose has to attach and can push results in either direction, per the NGSP laboratory standardization program.
• Hemoglobin variants: sickle cell trait and variants like HbC, HbE, and HbD, more common in people of African, Mediterranean, or Southeast Asian descent, interfere with some assay methods.
• Chronic kidney disease and dialysis: renal anemia and shortened red blood cell survival tend to make HbA1c underestimate true average glucose.
• Recent blood transfusion: introduces donor red blood cells with a different glycation history, distorting results for several weeks.
• Pregnancy: red blood cell turnover and iron status shift, one reason glucose tolerance testing rather than HbA1c is the standard screening tool there.

What to track over time

• The lab or method used for each draw, since assay methods vary and can shift results slightly between providers.
• The reference range printed on that specific lab report.
• Any conditions your doctor has flagged as potentially affecting accuracy, such as anemia or kidney function.
• The date of each test relative to other results, so the actual sequence is visible, not a single number in isolation.

What to ask your doctor

• What reference range did this lab use, and is that consistent with my last few results?
• Could anemia, a hemoglobin variant, or my kidney function be affecting how accurate this reading is for me?
• How does this result line up with my recent fasting glucose or CGM data, if I use one?
• What would you want to see before the next test, and on what timeline?
• Is a different test, like fructosamine, worth considering given my health history?

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