A1C, eAG & ADAG Formula Glossary
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Full bio →Lab reports, glucose meters, and continuous glucose monitors (CGMs) all describe blood sugar control differently. This glossary defines the terms behind those numbers and the exact formulas that connect them, so a result from any source can be translated into the others.
A1C (HbA1c)
A1C, also called HbA1c or glycated hemoglobin, is a blood test that measures the percentage of hemoglobin in red blood cells that has glucose attached to it. Because red blood cells live about three months, A1C reflects average blood sugar over roughly that period — not a single day. In the US, A1C is reported using the NGSP percentage standard (for example, 6.5%).
eAG — Estimated Average Glucose
eAG translates an A1C percentage into the same units shown on a home glucose meter or CGM (mg/dL in the US, mmol/L in most other countries). It exists because most people find “average glucose of 154 mg/dL” easier to picture than “A1C of 7.0%.” eAG is a calculated estimate, not a separate blood draw.
The ADAG Study and Formula
ADAG stands for A1C-Derived Average Glucose, a 2008 study (Nathan et al., published in Diabetes Care) that measured continuous glucose monitor data alongside A1C results across a large study population and found a strong linear correlation (r = 0.92). That study produced the formula labs and the ADA now use to report eAG next to A1C:
eAG (mg/dL) = 28.7 × A1C − 46.7
eAG (mmol/L) = eAG (mg/dL) ÷ 18.0182Reversed, the same formula estimates an A1C from a known average glucose (useful when you have weeks of CGM or meter data and want a rough lab-equivalent A1C):
A1C (%) = (eAG in mg/dL + 46.7) ÷ 28.7You can run either direction with the A1C calculator — enter A1C, mg/dL, mmol/L, or IFCC mmol/mol, and it converts to the other three automatically.
NGSP vs. IFCC — Two Reporting Scales, One Test
NGSP (National Glycohemoglobin Standardization Program) is the percentage scale used across the US — the ”%” you see on most American lab reports (for example, 7.0%).
IFCC (International Federation of Clinical Chemistry) is a newer, more specific measurement standard reported in millimoles per mole (mmol/mol) rather than a percentage. The UK, most of Europe, Australia, and several other countries report HbA1c in IFCC units, sometimes alongside the older percentage figure.
Both scales measure the exact same glycated hemoglobin molecule — they are not different tests, only different units, similar to Fahrenheit versus Celsius. The conversion is:
IFCC (mmol/mol) = (A1C% − 2.15) × 10.929
A1C% = (IFCC mmol/mol ÷ 10.929) + 2.15Quick Reference Table
| Term | What it means | Typical unit |
|---|---|---|
| A1C / HbA1c | Percent of hemoglobin bound to glucose over ~2–3 months | % (NGSP) |
| eAG | A1C translated into average glucose | mg/dL or mmol/L |
| IFCC | International standardized HbA1c unit | mmol/mol |
| ADAG | The 2008 study that produced the eAG formula | — (study name, not a unit) |
Why the Terminology Matters
Mixing up units is one of the most common sources of confusion when reading lab results from different countries or comparing a US A1C% to a UK mmol/mol value. For a worked walkthrough of specific A1C values converted into every format, see the A1C to average glucose conversion examples. To understand what each range of results actually indicates about diabetes risk, see ADA A1C diagnostic ranges explained.
A1C, eAG, and IFCC are all estimates of an underlying biological average — none of them capture minute-to-minute blood sugar swings. Always interpret a lab result together with a healthcare provider, particularly if it is a first-time diagnostic test.