Quick answer

A blood test measuring average blood sugar over the prior 2-3 months. Diabetes is diagnosed at HbA1c 6.5% or higher; prediabetes 5.7-6.4%. GLP-1 RAs typically lower HbA1c by 1.0-2.0 percentage points.

Full definition

Hemoglobin A1c is the percentage of hemoglobin molecules with glucose attached, reflecting average blood glucose exposure over the lifespan of red blood cells (about 120 days). It is the standard diagnostic test for diabetes (HbA1c 6.5%+), prediabetes (5.7-6.4%), and ongoing glycemic monitoring. GLP-1 RAs lower HbA1c through glucose-dependent insulin release, glucagon suppression, and weight loss — typical reductions are 1.0-1.5 percentage points for semaglutide and up to 2.3 points for tirzepatide at maximum doses.

Deep dive

HbA1c (Hemoglobin A1c): complete reference

Hemoglobin A1c (HbA1c, or simply A1c) is a blood test measuring the percentage of hemoglobin molecules in red blood cells that have glucose chemically attached (glycation). Because red blood cells circulate for approximately 120 days, A1c reflects average blood glucose over the preceding 2-3 months — providing a longer-term view than single fasting or post-meal glucose measurements. Diagnostic thresholds: normal (<5.7%), prediabetes (5.7-6.4%), diabetes (≥6.5%). For diabetes management, A1c targets are typically <7% for most adults, with adjustments for age, comorbidities, and hypoglycemia risk. GLP-1 receptor agonists produce substantial A1c reductions: semaglutide (Ozempic) ~1.5 percentage points, tirzepatide (Mounjaro 15mg) ~2.3 percentage points, dulaglutide (Trulicity) ~1.2 points, in pivotal trials. A1c is reported in two units: NGSP (%, US standard) and IFCC (mmol/mol, international). Conversion: A1c % = (IFCC + 2.15) / 0.0915. A1c can be misleading in conditions affecting red blood cell turnover (hemolytic anemia, recent transfusion, hemoglobinopathies) — alternative measures (fructosamine, CGM) may be used.

In practice: An A1c of 8.5% means your average blood glucose over the past 3 months was around 200 mg/dL — clearly above target. After starting Mounjaro 15mg, an A1c of 6.2% would represent average glucose around 130 mg/dL, much closer to non-diabetic range.
Clinical context: A1c is the primary marker for diabetes diagnosis and management. Most diabetes guidelines target <7% (some <6.5% for younger, healthier patients) with individualization for hypoglycemia risk.

Medications

HbA1c (Hemoglobin A1c) is most directly relevant to the following GLP-1 medications:

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GLP1Zoom glossary is educational reference. Definitions are summary interpretations of clinical sources and not a substitute for prescribing-information detail. Full disclaimer.

References