Warm sunrise scene with gentle hills. A smiling blood drop as a friendly character holds a small calendar page standing for the past eight to twelve weeks. Small glucose dots and red blood cells float around, illustrating that HbA1c captures the average of the last weeks.
8–12
weeks
Your blood sugar of recent weeks – in one value.
What HbA1c measures
HbA1c stands for glycated (“sugared”) haemoglobin. Haemoglobin is the red blood pigment that carries oxygen. When glucose circulates in the blood, some of it binds permanently to the haemoglobin. The higher the blood sugar over time, the greater the share of glycated haemoglobin.1
The value is given as a percentage of total haemoglobin (in Germany additionally in mmol/mol). Because red blood cells live about eight to twelve weeks, HbA1c reflects the average blood sugar of that period – hence the vivid name “blood-sugar memory”.1 The more recent weeks carry slightly more weight, because younger blood cells are more common.
Diagram: glucose in the blood binds permanently to haemoglobin, the red blood pigment, in red blood cells. The higher the blood sugar over time, the more haemoglobin is glycated. Since red blood cells live about 8 to 12 weeks, HbA1c reflects the average blood sugar of that period – the so-called blood-sugar memory.
How HbA1c forms
The blood-sugar memory of red blood cells
Low blood sugar
little glucose binds →
low HbA1c
→
over 8–12
weeks
High blood sugar
much glucose binds →
high HbA1c
Glucose (sugar)
red blood cell with haemoglobin
The binding is permanent. So the proportion of glycated haemoglobin reflects the average blood sugar of the last weeks.
The big practical advantage: for HbA1c you don’t need to be fasting, and it doesn’t swing from hour to hour like a single blood-sugar measurement.1 It smooths out the daily peaks and shows the trend.
Four friendly steps: first, go to the practice or lab without fasting. Second, a small blood draw. Third, the lab measures the proportion of glycated haemoglobin. Fourth, the doctor discusses the result with you. The test is straightforward and needs no special preparation.
How simple the test is
1
No fasting
needed
2
Small
blood draw
3
Lab measures
the value
4
Doctor’s
discussion
The thresholds: normal, increased risk, diabetes
The diagnostic thresholds are largely uniform internationally. The American Diabetes Association (ADA), the World Health Organization (WHO) and the German Diabetes Society (DDG) use the same central threshold for diagnosis.23
Scale of HbA1c in percent: below 5.7 percent normal (green), 5.7 to 6.4 percent raised risk or prediabetes (amber), from 6.5 percent diabetes range (red). In Germany the diagnosis usually requires confirmation by a second measurement.
HbA1c ranges: what the value means
Classification per ADA and the German Diabetes Society (DDG)
Normal
below 5.7%
Increased risk
5.7 – 6.4%
Diabetes range
6.5% and above
< 39 mmol/mol
39 – 47 mmol/mol
≥ 48 mmol/mol
Important for diagnosis
A single value of 6.5% and above usually is not enough in Germany: the diagnosis is generally made through a
second measurement or an additional glucose test. A value below 6.5% doesn’t reliably rule out diabetes.
The 6.5% threshold isn’t arbitrary: it’s based on large population studies in which the risk of diabetic retinal damage (retinopathy) rises markedly above this value.3 So it’s aligned with the risk of complications.
What your value means as average glucose
The percentage stays abstract if you deal with mg/dl or mmol/l every day. So HbA1c can be translated into an estimated average glucose (eAG). The basis is the ADAG study by Nathan and colleagues from 2008, which found a close linear relationship between HbA1c and actually measured average glucose in 507 people.4
Conversion table per the ADAG formula (Nathan et al., 2008): 6 percent HbA1c corresponds to about 126 mg/dl, 6.5 percent about 140 mg/dl, 7 percent about 154 mg/dl, 8 percent about 183 mg/dl, 9 percent about 212 mg/dl. Formula: eAG in mg/dl equals 28.7 times HbA1c minus 46.7.
What HbA1c means as average glucose
Estimated average glucose (eAG) per the ADAG formula
eAG (mg/dl) = 28.7 × HbA1c − 46.7
HbA1c
eAG (mg/dl)
eAG (mmol/l)
6.0%1267.0
6.5%1407.8
7.0%1548.6
8.0%18310.2
9.0%21211.8
Values rounded. The formula describes a statistical average – individually the value can differ.
Why the value matters: the evidence
Evidence level: established (large randomised trials and prospective cohorts)
HbA1c isn’t just a snapshot but a proven risk marker. The UK Prospective Diabetes Study (UKPDS 35), a large prospective study in type 2 diabetes, showed: each 1-percentage-point reduction in HbA1c was associated with a 37% lower risk of microvascular complications (eyes, kidneys, nerves), a 21% lower risk of diabetes-related deaths and a 14% lower risk of heart attack.5 Notably, there was no threshold – the lowest risk was at values in the normal range.5
In type 1 diabetes, the Diabetes Control and Complications Trial (DCCT) demonstrated the same relationship: more intensive blood-sugar lowering markedly reduced microvascular complications.6 Together these are two of the most important diabetes studies ever – and the reason HbA1c is central to monitoring today.
When HbA1c misleads
Evidence level: established, with variable effect size in individual cases
This is the most important and most often overlooked part. Because HbA1c depends on red blood cells, anything that changes their lifespan or number distorts the result – independent of the actual blood sugar.7
When red blood cells live longer, more glycated haemoglobin accumulates. The most significant case is iron-deficiency anaemia: a systematic review found elevated HbA1c values here that fell again after iron treatment – sometimes by more than a percentage point, without any change in blood sugar.7 Vitamin B12 or folate deficiency can also skew the value upward.79
If the lifespan of the blood cells shortens, there’s less time for glycation – HbA1c comes out too low. This affects haemolytic anaemias, acute or chronic blood loss, an enlarged spleen and the period after a blood transfusion.78 Advanced kidney disease can also distort the value.10
In pregnancy, HbA1c is not suitable for diagnosing gestational diabetes – an oral glucose tolerance test (OGTT) is used here.8 Haemoglobin variants (e.g. in certain inherited blood disorders) can lead to falsely high or low values depending on the lab method.7
Friendly summary card with the four key points: HbA1c is the average of the last eight to twelve weeks. From 6.5 percent is the diabetes range, but a doctor always confirms the diagnosis. Every percentage point lower reduces the risk of complications. Anaemia and other blood changes can skew the value.
HbA1c at a glance
Four things to remember
📅
Average of 8–12 weeks
Your “blood-sugar memory”, not a snapshot.
🎯
From 6.5%, the diabetes range
The diagnosis is always made by a doctor.
📉
Every percentage point counts
Lower HbA1c = lower risk of complications.
⚠️
Can mislead
Anaemia & co. can skew the value.
When in doubt: interpret the value and your own measurements together with your doctor.
Key takeaways
- HbA1c shows the average blood sugar of the last roughly 8–12 weeks.
- Thresholds: below 5.7% normal, 5.7–6.4% increased risk, from 6.5% the diabetes range – a second measurement confirms the diagnosis medically.
- The ADAG formula translates the value into a familiar average glucose (eAG).
- Per UKPDS, each percentage point lower markedly reduces the risk of complications – with no clear threshold.
- Anaemia, blood loss, pregnancy and haemoglobin variants can skew the value.
Sources
Sources as of: 21 Jul 2026.
- HbA1c: formation, glycated haemoglobin, ~8–12 week window; no fasting needed. Reference. mayocliniclabs.com
- Diagnostic HbA1c thresholds (ADA/WHO); ≥6.5% for diagnosis. Reference. emedicine.medscape.com
- 6.5% threshold aligned with retinopathy risk; DDG classification/diagnostics. ddg.info
- ADAG study (Nathan et al., 2008): linear HbA1c–average-glucose relationship (eAG). Diabetes Care. diabetesjournals.org
- UKPDS 35: each 1% lower HbA1c → 37% fewer microvascular complications, 14% fewer heart attacks; no threshold. BMJ/PubMed. pubmed.ncbi.nlm.nih.gov
- DCCT (type 1): intensive glucose lowering markedly reduces microvascular complications. PubMed. pubmed.ncbi.nlm.nih.gov
- HbA1c pitfalls: iron/B12/folate deficiency raise it; haemolysis, blood loss, variants lower/skew it. Systematic review. pmc.ncbi.nlm.nih.gov
- Haemoglobin variants and conditions that limit HbA1c reliability; pregnancy uses OGTT. Review. frontierspartnerships.org
- Radin M.S. (2014, and others): Limitations of haemoglobin A1c in the management of type 2 diabetes. Confounders: iron/B12/folate deficiency (falsely high), haemolysis/blood loss/splenomegaly (falsely low), haemoglobinopathies; iron repletion lowers HbA1c ~1.2%. PMC. pmc.ncbi.nlm.nih.gov/PMC7021345
- Overview of confounders including pregnancy (OGTT instead of HbA1c), transfusion, kidney disease. Frontiers / British Journal of Biomedical Science (2024). frontierspartnerships.org