What is Understanding HbA1c — What It Means and How to Use It?

What HbA1c measures, how it’s used for diagnosis and monitoring, typical targets, limitations, and how it relates to Time-in-Range.

Diabetes

Understanding HbA1c — What It Means and How to Use It

2025-08-20 • Updated 2025-09-13

Intro

HbA1c (also called A1C) reflects your average blood glucose over ~3 months by measuring how much glucose is attached to haemoglobin in red blood cells. It’s used to diagnose diabetes and prediabetes and to track long-term control. :contentReference[oaicite:0]{index=0}

Key Points

HbA1c is an average of the last ~8–12 weeks, not a spot glucose reading. :contentReference[oaicite:1]{index=1}
Diagnosis (ADA/WHO): diabetes at ≥6.5% (48 mmol/mol); prediabetes 5.7–6.4% (ADA). Use standardised assays. :contentReference[oaicite:2]{index=2}
Typical target for many adults with diabetes: <7% (individualise). :contentReference[oaicite:3]{index=3}
Pair HbA1c with CGM Time-in-Range (TIR) for day-to-day quality and safety. Aim ≥70% TIR (70–180 mg/dL / 3.9–10.0 mmol/L), with <4% below 70 mg/dL. :contentReference[oaicite:4]{index=4}
HbA1c can be misleading with anaemia, haemoglobin variants, kidney disease, pregnancy, recent transfusion, or rapid RBC turnover. :contentReference[oaicite:5]{index=5}

What exactly does HbA1c measure?

Glucose slowly binds to haemoglobin inside red blood cells. Because RBCs live ~3 months, the percentage that is glycated (HbA1c) tracks your mean exposure to glucose over that period. Higher average glucose → higher HbA1c. :contentReference[oaicite:6]{index=6}

Using HbA1c for Diagnosis

Diabetes: A1C ≥6.5% (48 mmol/mol), confirmed (or with classic symptoms and random glucose).
Prediabetes (ADA): A1C 5.7–6.4% (39–46 mmol/mol).
Use a lab method aligned to IFCC/NGSP standards; when results conflict with glucose tests or clinical picture, repeat/confirm with another test. :contentReference[oaicite:7]{index=7}

Targets & Testing Frequency

Targets (individualise): Many non-pregnant adults aim for A1C <7%; tighter or looser goals depend on age, comorbidity, and hypoglycaemia risk. :contentReference[oaicite:8]{index=8}
How often to test:
- Every 6 months if stable and at goal.
- Every 3 months if therapy changed or goals not met. (Per ADA Standards.) :contentReference[oaicite:9]{index=9}

HbA1c ↔ Estimated Average Glucose (eAG)

The ADAG formula converts A1C to an estimated average glucose:

eAG (mg/dL) = 28.7 × A1C − 46.7
eAG (mmol/L) = (28.7 × A1C − 46.7) ÷ 18 :contentReference[oaicite:10]{index=10}

HbA1c (%)	eAG (mg/dL)	eAG (mmol/L)
5	97	5.4
6	125	7.0
7	154	8.6
8	183	10.2
9	212	11.8
10	240	13.4
11	269	14.9
12	298	16.5

eAG is an estimate. Day-to-day swings, hypoglycaemia exposure, and overnight control may not be obvious from HbA1c alone.

HbA1c + Time-in-Range (TIR): Better Together

HbA1c shows the average, while TIR (from CGM) shows how often you’re in the target band and how much time is spent low or high.

General adult targets: TIR ≥70% (70–180 mg/dL / 3.9–10.0 mmol/L); TBR <4% (<70 mg/dL) and <1% (<54 mg/dL). Individualise for frailty/pregnancy. :contentReference[oaicite:11]{index=11}

When HbA1c Can Mislead

Consider alternatives (e.g., fructosamine/glycated albumin) or rely more on CGM metrics when any of the below apply:

Anaemia (iron deficiency can raise; haemolysis can lower), recent transfusion, erythropoietin, dialysis, CKD.
Haemoglobin variants (e.g., HbS, HbC) — some assays are affected.
Pregnancy (altered RBC lifespan; not used to diagnose gestational diabetes). :contentReference[oaicite:12]{index=12}

FAQ

Is HbA1c the same as my “average” on CGM?
They usually correlate, but CGM averages and HbA1c can diverge due to sensor use time, day-night variability, anaemia, or RBC lifespan. Use both HbA1c and TIR. :contentReference[oaicite:13]{index=13}

My HbA1c looks “too good” for my meter readings — why?
Check for factors that lower HbA1c artificially (e.g., haemolysis, recent blood loss/transfusion). :contentReference[oaicite:14]{index=14}

What should my A1C target be?
Many adults aim <7%; older adults or those at high hypoglycaemia risk may use a less stringent goal. Discuss with your clinician. :contentReference[oaicite:15]{index=15}