HbA1c

The share of hemoglobin with glucose attached, reflecting average blood glucose over roughly the previous two to three months.

What it measures

Glucose attaches to hemoglobin inside red blood cells slowly and irreversibly. Because red cells live about three months, the percentage of glycated hemoglobin gives a weighted average of blood glucose over that period, skewed toward the most recent weeks.

That is its strength. A single fasting glucose is a snapshot that a bad night or a stressful morning can distort. HbA1c is much harder to game.

Thresholds

The American Diabetes Association places prediabetes at 5.7 to 6.4 percent and diabetes at 6.5 percent or above, confirmed on a repeat test. Below 5.7 is considered normal, though risk within that band is not flat and the lower part of the range is associated with better outcomes.

An HbA1c in the prediabetes range is the point at which intervention has the best evidence behind it, and it is often the earliest number on a standard panel that moves.

When it lies

Anything that changes red cell lifespan changes the result. Iron deficiency anemia and recent blood loss can raise it. Hemolysis, recent transfusion, pregnancy, chronic kidney disease and hemoglobin variants can lower it or make it uninterpretable. Some variants are common in people of African, Mediterranean and Southeast Asian ancestry.

It also averages away variability. Two people with the same HbA1c can have very different post-meal spikes, which is why fasting insulin, HOMA-IR or continuous glucose data add information that HbA1c cannot.

Reference ranges

  • Normal: below 5.7 percent. ADA Standards of Care. Diagnostic threshold, not a personal target.
  • Prediabetes: 5.7 to 6.4 percent. ADA Standards of Care.
  • Diabetes: 6.5 percent or above. ADA Standards of Care, confirmed on repeat testing.

HbA1c is one of the markers we recommend before and during metabolic work, and lab work is optional.

References

  1. American Diabetes Association. Standards of Care in Diabetes: classification and diagnosis.
  2. Nathan DM, et al. Translating the A1C assay into estimated average glucose values. Diabetes Care. 2008.
  3. Radin MS. Pitfalls in hemoglobin A1c measurement: when results may be misleading. J Gen Intern Med. 2014.