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A hemoglobin A1C level above 6.5% indicates that a person has diabetes mellitus. The A1C test measures the average blood glucose levels over the past two to three months, and a result of 6.5% or higher confirms a diagnosis of diabetes according to the standards set by the American Diabetes Association. This level indicates that blood glucose has been consistently elevated, signaling that the body is not effectively managing glucose either due to insufficient insulin production, insulin resistance, or both.

For context, a level between 5.7% and 6.4% generally indicates prediabetes, which is a state where blood glucose levels are higher than normal but not yet high enough to be classified as diabetes. Type 1 diabetes refers to a specific autoimmune condition that typically arises in younger individuals and leads to absolute insulin deficiency, but it is characterized by higher A1C levels just like any form of diabetes. Insulin resistance can occur in people with type 2 diabetes or prediabetes, but it is not a direct diagnosis indicated solely by A1C levels above 6.5%.