Understanding HbA1c

HbA1c is a blood test your doctor orders to measure your average blood glucose over the past 2–3 months. It is one of the most important tests for diagnosing and monitoring diabetes and prediabetes. This app's GMI metric estimates this number from your readings — but it is important to understand both what the test measures and what its limitations are.

How It Works

HbA1c measures the percentage of your red blood cells that have glucose attached to them (glycated hemoglobin). Because red blood cells live for about 90 days, the test reflects your average glucose control over that entire period — not just the day of the test.

The relationship between HbA1c and your average daily glucose is:

Estimated Average Glucose (eAG) = 28.7 × HbA1c − 46.7

For example: HbA1c of 6.0% → eAG = (28.7 × 6.0) − 46.7 = 172.2 − 46.7 = 125.5 mg/dL

This means a person with an HbA1c of 6.0% has an average glucose of roughly 126 mg/dL across all hours of the day and night.

Your Target

• Below 5.7% — Normal
• 5.7% to 6.4% — Prediabetic range
• 6.5% and above — Diabetic range (confirmed on two separate tests)

For prediabetes management, the goal is to bring your HbA1c below 5.7%.

Why This Matters

A single glucose reading tells you what your blood sugar was at that moment. HbA1c tells you about the broader pattern over months. A person can have normal fasting readings but elevated HbA1c if they regularly have large undetected post-meal spikes. This is why tracking post-meal readings in this app captures information that a fasting-only measurement would miss.

Important: The GMI in this app is an estimate, not a laboratory measurement. It can differ from your actual HbA1c by 0.3–0.5% or more. Your doctor's lab test is the definitive measure for medical decisions. Use GMI to track your direction between appointments, not as a replacement for testing.

What You Can Do

• Get your HbA1c tested at your doctor's clinic every 3 months when first managing prediabetes, or as your doctor recommends.
• Use the GMI trend in this app to get a monthly directional signal between those appointments.
• If your GMI is rising even slightly over 4–6 weeks, bring it up at your next appointment rather than waiting for the annual check.

Based on: Nathan et al., Diabetes Care 2008; American Diabetes Association Standards of Care 2023