Understanding Your HbA1c: What the Number Really Means

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HbA1c is arguably the most important number in diabetes management — yet it is widely misunderstood by patients and sometimes misinterpreted in clinical practice. This article explains what HbA1c actually measures, what your target should be, when the test is unreliable, and why a single number is no longer the whole story in modern diabetes care.

What Is HbA1c?

HbA1c — haemoglobin A1c, or glycated haemoglobin — reflects the proportion of haemoglobin molecules in your red blood cells that have become irreversibly bound to glucose molecules through a process called glycation. Because red blood cells survive for approximately 90–120 days, HbA1c provides a weighted average of blood glucose over that period, with the most recent 6–8 weeks contributing most heavily to the result.

In the United Kingdom and Ireland, HbA1c is reported in mmol/mol (the IFCC unit), which replaced the older percentage (DCCT/NGSP) notation. Many patients and some clinical letters still use both — the table below provides a conversion reference for the most clinically relevant values:

mmol/mol (IFCC)	% (DCCT)	Clinical significance
42 mmol/mol	6.0%	Upper limit of normal
47 mmol/mol	6.4%	Top of prediabetes range
48 mmol/mol	6.5%	Diabetes diagnosis threshold
53 mmol/mol	7.0%	Common treatment target (T2DM on hypoglycaemia-risk agents)
58 mmol/mol	7.5%	Acceptable in elderly or high hypoglycaemia risk
69 mmol/mol	8.5%	Suboptimal — increased complications risk
86 mmol/mol	10.0%	Poor control — urgent review needed

Diagnostic Thresholds

Per NICE NG28 (updated 2022) and ADA 2024 Standards of Care:

• ≥48 mmol/mol (6.5%) on two occasions in an asymptomatic person, or once in a person with classic symptoms of hyperglycaemia (polyuria, polydipsia, unexplained weight loss) → diabetes diagnosis confirmed
• 42–47 mmol/mol (6.0–6.4%) → non-diabetic hyperglycaemia (prediabetes / impaired glucose regulation). This is not a "borderline" finding to be reassured about — around 5–10% of people in this range develop type 2 diabetes per year. Structured lifestyle intervention can reduce this risk by up to 58%.
• <42 mmol/mol → normal

Treatment Targets — Why "One Size" Does Not Fit All

The single most important advance in diabetes management over the past decade has been the move towards individualised HbA1c targets. The evidence clearly shows that the risks of intensive glucose control (primarily hypoglycaemia and its consequences) can outweigh the benefits in certain populations.

NICE guidance currently recommends:

• 48 mmol/mol (6.5%): the initial target for most people with newly diagnosed type 2 diabetes managed with lifestyle modification and metformin alone, if achievable without hypoglycaemia or significant treatment burden
• 53 mmol/mol (7.0%): when the person is taking a medication associated with hypoglycaemia (sulphonylurea, insulin), has established CVD, cognitive impairment, or is older with comorbidities where tight control poses risk
• 58–64 mmol/mol (7.5–8.0%): appropriate in frail older patients, those in care settings, patients with limited life expectancy, or where the burden of treatment outweighs likely benefit
• Type 1 diabetes: ≤48 mmol/mol (6.5%) per NICE NG17, with continuous glucose monitoring (CGM) increasingly the primary tool for self-management and monitoring

The ADA 2024 and EASD 2023 joint consensus emphasises that glycaemic targets must be co-developed with the patient, taking into account their values, preferences, burden of treatment, hypoglycaemia risk, life expectancy, and likelihood of benefit over a realistic time horizon. A healthy 45-year-old with newly diagnosed T2DM should aim for ≤48 mmol/mol; a 78-year-old with heart failure, cognitive decline, and recurrent falls may be safely and appropriately managed at 64 mmol/mol.

When HbA1c Is Unreliable — A Critical Limitation

HbA1c depends on two assumptions: that red blood cells have a normal lifespan (~120 days) and that haemoglobin structure is normal. When either is altered, HbA1c does not accurately reflect average glucose. This is clinically critical and frequently overlooked.

Conditions Causing Falsely LOW HbA1c (glucose appears better than it is)

• Haemolytic anaemia (autoimmune, sickle cell crisis, microangiopathic) — red cells destroyed faster, less time for glycation
• Haemoglobinopathies — HbSS (sickle cell), HbC, HbE, HbD — interfere with HPLC assay methodology used by most laboratories
• Recent blood transfusion — donor red cells, not glycated by the patient's glucose, dilute the result
• Iron deficiency treatment — rapid production of young reticulocytes with shorter glycation exposure
• Chronic liver disease — reduced red cell survival

Conditions Causing Falsely HIGH HbA1c (glucose appears worse than it is)

• Iron deficiency anaemia — iron-deficient red cells survive longer, accumulating more glycation
• Vitamin B12 or folate deficiency
• Uraemia / advanced CKD — carbamylated haemoglobin cross-reacts with some assays
• Asplenia or hyposplenia — prolonged red cell survival

Alternative Tests When HbA1c Is Unreliable

• Fructosamine — reflects glucose control over the preceding 2–3 weeks. Useful when rapid changes are occurring or HbA1c is unreliable. Less standardised internationally.
• Continuous glucose monitoring (CGM) — the gold standard for real-time glucose data. Provides time-in-range (TIR), time below range (TBR), glucose variability — far richer information than HbA1c alone.

Beyond HbA1c — The Time-in-Range Revolution

HbA1c has a fundamental and underappreciated limitation: it is an average. Two patients with identical HbA1c of 58 mmol/mol may have profoundly different glucose profiles — one with stable, modestly elevated glucose; the other swinging between dangerous hypoglycaemia and postprandial hyperglycaemia. The average is the same; the clinical situation is entirely different.

Continuous glucose monitoring has transformed diabetes management. The 2023 international consensus targets for people with type 2 diabetes are:

• Time-in-Range (TIR): >70% of readings between 3.9–10.0 mmol/L
• Time Below Range (TBR): <4% of time below 3.9 mmol/L; <1% below 3.0 mmol/L
• Time Above Range (TAR): <25% above 10.0 mmol/L; <5% above 13.9 mmol/L
• Glucose Management Indicator (GMI): estimated HbA1c derived from CGM mean glucose — useful cross-reference but not a substitute for laboratory HbA1c in diagnosis

For older or high-risk patients where hypoglycaemia is the primary concern, TBR targets are the most critical metric. A clinician who focuses only on HbA1c and misses recurring hypoglycaemia at 02:00 is missing the most dangerous part of that patient's glucose profile.

HbA1c in Prediabetes — The Missed Opportunity

The prediabetes range (42–47 mmol/mol) represents the highest-yield opportunity in diabetes prevention. The landmark Diabetes Prevention Programme (DPP) and Finnish Diabetes Prevention Study demonstrated that structured lifestyle intervention — achieving 5–7% weight loss and 150 minutes of moderate physical activity per week — reduces progression from prediabetes to type 2 diabetes by 58%. Metformin reduces progression by 31%.

Patients with HbA1c in the prediabetes range should not simply be "rechecked in a year." They should receive:

• Clear explanation of the clinical significance and trajectory
• Structured lifestyle advice or formal referral to a prevention programme
• Assessment and management of other cardiometabolic risk factors
• Annual monitoring at minimum
• Consideration of metformin if lifestyle modification is insufficient

When to Seek Specialist Review

• HbA1c persistently >75 mmol/mol (9.0%) despite optimised oral therapy
• Any HbA1c >86 mmol/mol (10.0%)
• Recurrent or severe hypoglycaemia at any HbA1c level
• Suspected haemoglobin variant or haematological condition causing unreliable HbA1c
• Newly diagnosed diabetes in a person under 30 — consider type 1, LADA (latent autoimmune diabetes in adults), or MODY (maturity-onset diabetes of the young)
• Diabetes complicating pregnancy or preconception planning — HbA1c targets in pregnancy are tighter (<48 mmol/mol ideally <43 mmol/mol), and management is complex
• Rapid deterioration in glycaemic control suggesting a secondary cause (new steroid prescription, intercurrent illness, endocrine condition such as Cushing's syndrome or acromegaly)
• Significant weight gain despite good lifestyle adherence — consider insulin resistance, medication side effects, or endocrine cause