HbA1c is a key biomarker in diabetes care, but is HbA1c an advanced glycation end product? This is a clinically important distinction. HbA1c — or glycated haemoglobin — forms through non-enzymatic glycation of haemoglobin and is classified as an Amadori product, representing an early stage in the glycation cascade. Advanced glycation end products (AGEs) are distinct molecules that accumulate over months to years through further complex reactions. Understanding where HbA1c sits within this biochemical pathway helps clarify both its value and its limitations as a marker of long-term glycaemic exposure and diabetic complication risk.

What Is HbA1c and How Does It Form?

HbA1c forms when glucose binds to haemoglobin via non-enzymatic glycation, producing a stable Amadori product — not an advanced glycation end product — that reflects average blood glucose over two to three months.

HbA1c, or glycated haemoglobin, is a well-established biomarker used in the diagnosis and monitoring of diabetes mellitus. It reflects the average blood glucose concentration over the preceding two to three months — a timeframe that corresponds broadly to the lifespan of a red blood cell. Importantly, HbA1c represents a weighted average, with glucose levels from the most recent four weeks contributing more heavily than those from earlier in the period. The measurement is expressed as a percentage or in millimoles per mole (mmol/mol), with the latter being the standard unit used across NHS laboratories in the United Kingdom, in line with WHO 2011 guidance on the use of HbA1c in the diagnosis of diabetes mellitus.

The formation of HbA1c begins with a process called non-enzymatic glycation, specifically the Maillard reaction. When blood glucose levels are elevated, glucose molecules bind spontaneously to the N-terminal valine residue of the beta chain of haemoglobin. This initial, reversible attachment forms a Schiff base (aldimine), which then undergoes a slower, irreversible rearrangement known as the Amadori rearrangement, producing a stable ketoamine product — this is HbA1c.

It is important to note that HbA1c is technically classified as an Amadori product, which represents an early-stage glycation product. This distinguishes it from advanced glycation end products (AGEs), which are formed through further, more complex chemical reactions over a longer period. So while HbA1c does arise from glycation, it sits at an earlier point in the glycation cascade rather than at the advanced end. Understanding this distinction is clinically meaningful, as it helps clarify what HbA1c can and cannot tell us about the broader biochemical consequences of chronic hyperglycaemia.

Understanding Advanced Glycation End Products (AGEs)

AGEs are distinct from HbA1c; they form through prolonged glycation, oxidation, and cross-linking reactions over months to years, accumulating in tissues and driving diabetic complications via RAGE receptor signalling.

Advanced glycation end products (AGEs) are a heterogeneous group of molecules formed when proteins, lipids, or nucleic acids undergo prolonged non-enzymatic glycation and oxidation. Unlike HbA1c — which is an Amadori product formed over weeks — AGEs accumulate over months to years, making them markers of cumulative glycaemic exposure and oxidative stress rather than short-term glucose control.

The biochemical pathway from Amadori products to AGEs involves a series of further reactions, including oxidation, dehydration, and cross-linking. Key AGEs include:

• Carboxymethyl-lysine (CML) — one of the most studied AGEs, found in tissues and plasma

• Pentosidine — a cross-linking AGE associated with collagen ageing

• Methylglyoxal-derived AGEs — linked to vascular and neurological damage

AGEs exert their harmful effects through two main mechanisms. First, they directly alter the structural and functional properties of proteins — particularly long-lived proteins such as collagen, myelin proteins, and lens crystallins — by forming irreversible cross-links. Second, they interact with specific cell-surface receptors known as RAGE (Receptor for AGEs), triggering inflammatory signalling pathways, oxidative stress, and endothelial dysfunction.

In clinical practice, AGE accumulation has been associated with the development of diabetic microvascular and macrovascular complications, including nephropathy, retinopathy, neuropathy, and cardiovascular disease, though the precise contribution of AGEs relative to other pathways continues to be investigated. Skin autofluorescence (SAF) is an emerging non-invasive technique used in research settings to estimate tissue AGE burden; however, it is not recommended by NICE for routine NHS clinical care, and measuring AGEs or SAF does not currently form part of standard diabetes management pathways in the UK.

The distinction between HbA1c and AGEs is therefore not merely academic — it has real implications for understanding why some patients may develop complications despite apparently well-controlled HbA1c levels.

Clinical Relevance of Glycation in Long-Term Diabetes Care

HbA1c does not fully capture cumulative glycaemic burden driving AGE accumulation; complications can develop despite target HbA1c values, particularly in long-standing diabetes, and several conditions can render HbA1c unreliable.

The relationship between glycation and diabetic complications underscores why glycaemic control remains a cornerstone of diabetes management. Landmark trials including the DCCT/EDIC (type 1 diabetes) and UKPDS (type 2 diabetes) demonstrated that sustained improvements in glycaemic control significantly reduce the risk of microvascular and macrovascular complications. While HbA1c provides a reliable snapshot of medium-term glucose exposure, it does not fully capture the cumulative glycaemic burden that drives AGE accumulation and tissue damage. This is particularly relevant in patients with long-standing type 1 or type 2 diabetes, where complications may develop or progress despite HbA1c values within target range.

Several factors can affect the reliability of HbA1c as a glycaemic marker, and there are specific circumstances in which HbA1c should not be used for diagnosis or may give misleading results:

• Haemoglobinopathies (e.g., sickle cell trait, thalassaemia) can falsely lower or raise HbA1c readings

• Haemolytic anaemia shortens red cell lifespan, reducing HbA1c independently of glucose levels

• Iron deficiency anaemia may falsely elevate HbA1c

• Advanced chronic kidney disease (CKD) and certain medications can also interfere with results

• Pregnancy — HbA1c is not recommended for diagnosing gestational diabetes or monitoring in pregnancy

• Children and young people — diagnosis should follow specialist assessment

• Suspected type 1 diabetes or rapid-onset symptoms — fasting plasma glucose (FPG) or oral glucose tolerance test (OGTT) are preferred

• Recent acute illness or blood transfusion — results may be unreliable

In such cases, alternative markers such as fructosamine (a measure of glycated serum proteins over approximately two to three weeks) or glycated albumin may be considered, though these are also Amadori products rather than AGEs, and are not routinely recommended by NICE. Where HbA1c is unsuitable for diagnosis, FPG or OGTT should be used in accordance with WHO 2011 guidance.

From a pathophysiological standpoint, AGE accumulation is thought to contribute to the stiffening of arterial walls, glomerular basement membrane thickening, and peripheral nerve damage — hallmarks of long-term diabetic complications, though the mechanisms of diabetic neuropathy also involve axonal loss and ischaemia alongside demyelination. Clinicians should therefore consider the totality of a patient's glycaemic history, not just current HbA1c, when assessing complication risk.

Lifestyle factors such as smoking and oxidative stress may independently accelerate AGE formation. Some research suggests that dietary AGE intake — particularly from highly processed or charred foods — may also contribute, though the human evidence remains mixed and this is not reflected in current NICE or NHS dietary recommendations for diabetes management. Patients should follow standard NHS dietary guidance for diabetes rather than attempting to restrict dietary AGEs specifically.

NHS Guidance on HbA1c Testing and Diabetes Management

NICE recommends HbA1c testing every three to six months when treatment is adjusted and at least annually once stable, with a diagnostic threshold of ≥48 mmol/mol; CGM is recommended as an adjunct, not a replacement.

In the United Kingdom, HbA1c testing is governed by guidance from NICE, NHS England, and the World Health Organisation (WHO). According to NICE guideline NG28 (Type 2 diabetes in adults) and NG17 (Type 1 diabetes in adults), HbA1c should be measured every three to six months when treatment is being adjusted, and at least annually once stable targets are achieved.

Diagnostic thresholds for diabetes, as adopted by NHS laboratories in line with WHO 2011 guidance, are:

• HbA1c ≥ 48 mmol/mol (6.5%) — diagnostic of diabetes (confirmed on two occasions if asymptomatic, or once if symptomatic)

• HbA1c 42–47 mmol/mol (6.0–6.4%) — indicates non-diabetic hyperglycaemia (prediabetes), warranting lifestyle intervention and annual review

People identified with non-diabetic hyperglycaemia should be referred to the NHS Diabetes Prevention Programme (NHS DPP), in line with NICE guideline PH38, which provides structured lifestyle support to reduce the risk of progression to type 2 diabetes.

NICE recommends individualised HbA1c targets, typically:

• 48 mmol/mol (6.5%) for most adults with type 2 diabetes managed by lifestyle or metformin alone (NICE NG28)

• 53 mmol/mol (7.0%) for those on medications that carry a risk of hypoglycaemia (NICE NG28)

• 48 mmol/mol (6.5%) or lower, if safely achievable without problematic hypoglycaemia, for adults with type 1 diabetes (NICE NG17)

HbA1c alone does not capture glycaemic variability or hypoglycaemic episodes. Continuous glucose monitoring (CGM) is increasingly recommended as an adjunct to HbA1c — particularly for people with type 1 diabetes, for whom NICE NG17 recommends CGM be offered routinely, and for insulin-treated type 2 diabetes in certain circumstances per NICE NG28. CGM complements rather than replaces HbA1c, which remains a key laboratory measure within NHS diabetes care.

HbA1c should not be used for diagnosis in the circumstances listed in the section above (pregnancy, children, suspected type 1 diabetes, haemoglobinopathies, advanced CKD, recent transfusion, or acute illness). In these situations, FPG or OGTT should be used.

When to seek medical advice — patient safety guidance:

Contact your GP or diabetes care team if you experience:

• Persistent symptoms of hyperglycaemia (excessive thirst, frequent urination, fatigue, blurred vision)

• Unexplained weight loss

• Signs of hypoglycaemia (shakiness, sweating, confusion, palpitations)

Seek emergency care immediately (call 999 or go to A&E) if you or someone else has:

• Symptoms of diabetic ketoacidosis (DKA) — vomiting, abdominal pain, rapid breathing, fruity-smelling breath, or reduced consciousness (particularly in type 1 diabetes)

• Symptoms of hyperosmolar hyperglycaemic state (HHS) — extreme thirst, confusion, or drowsiness with very high blood glucose (more common in type 2 diabetes)

• Severe hypoglycaemia — loss of consciousness, seizure, or inability to treat with oral glucose

Further information is available on the NHS website for DKA, HHS, and hypoglycaemia.

Whilst HbA1c is not itself an advanced glycation end product, its measurement remains the most practical and validated tool available within NHS practice for guiding diabetes treatment decisions and reducing the long-term risk of AGE-mediated complications.

Frequently Asked Questions