Abnormal HbA1c ICD-10 coding is an important consideration for clinicians, coders, and healthcare professionals working across NHS settings. HbA1c — glycated haemoglobin — is the primary blood test used in the UK to diagnose diabetes and monitor long-term glycaemic control, with results expressed in mmol/mol under IFCC standardisation. Whether a result falls in the non-diabetic hyperglycaemia range (42–47 mmol/mol) or meets the diagnostic threshold for type 2 diabetes (48 mmol/mol and above), accurate clinical coding, appropriate interpretation, and timely management are essential to reducing the risk of serious long-term complications.

What Is an Abnormal HbA1c and Why Does It Matter?

An abnormal HbA1c reflects sustained disruption to blood glucose control; under NHS and NICE guidance, 42–47 mmol/mol indicates non-diabetic hyperglycaemia and 48 mmol/mol or above is diagnostic of type 2 diabetes.

HbA1c, or glycated haemoglobin, is a blood test that reflects average blood glucose levels over the preceding two to three months. It measures the proportion of haemoglobin molecules that have glucose attached to them, expressed in millimoles per mole (mmol/mol) in the UK, following adoption of the IFCC standardisation. Because it captures a longer-term picture of glycaemic control than a fasting glucose test, it is widely used both to diagnose diabetes and to monitor how well blood sugar is being managed in people already living with the condition.

An abnormal HbA1c result — whether unexpectedly high or, in certain clinical contexts, unusually low — carries significant implications for patient health. Persistently elevated HbA1c is associated with an increased risk of microvascular complications such as diabetic retinopathy, nephropathy, and peripheral neuropathy, as well as macrovascular events including myocardial infarction and stroke. Conversely, an HbA1c that is lower than expected in a person on glucose-lowering therapy may signal a risk of hypoglycaemia or an underlying haematological condition affecting red blood cell turnover.

Understanding what constitutes an abnormal result is therefore clinically important. According to NHS and NICE guidance (NICE NG28), the following thresholds are used:

• Below 42 mmol/mol: Normal (non-diabetic range)

• 42–47 mmol/mol: Non-diabetic hyperglycaemia (NDH) — sometimes referred to as prediabetes

• 48 mmol/mol and above: Diagnostic of type 2 diabetes (when confirmed on a second test in asymptomatic individuals, or on a single result in the presence of symptoms)

The term non-diabetic hyperglycaemia (NDH) is preferred in UK clinical practice; the term "prediabetes" is also used by the NHS but is less precise. Recognising an abnormal HbA1c early enables timely intervention, lifestyle modification, and, where necessary, pharmacological management to reduce long-term complications.

ICD-10 Codes Used for Abnormal HbA1c Results in the UK

R73.9 (hyperglycaemia, unspecified) is the most relevant ICD-10 code for an abnormal HbA1c without a confirmed diagnosis; confirmed diabetes is coded under E10–E13 in NHS secondary care.

The International Classification of Diseases, 10th Revision (ICD-10), is the coding system used across NHS secondary care settings to classify diagnoses, symptoms, and clinical findings. It is important to note that UK primary care uses SNOMED CT (Systematised Nomenclature of Medicine – Clinical Terms) rather than ICD-10; clinical concepts such as "non-diabetic hyperglycaemia" or "impaired glucose regulation" are recorded using SNOMED CT codes in GP systems. ICD-10 coding in secondary care is carried out by trained clinical coders following the NHS England National Clinical Coding Standards (ICD-10 5th Edition).

A key principle of UK clinical coding is that coders record documented clinical diagnoses or problems — they do not code raw laboratory values in isolation. An abnormal HbA1c result would only be coded if a clinician has documented a corresponding clinical finding or diagnosis.

The most relevant ICD-10 codes in this context are:

• R73.0 – Abnormal result of glucose tolerance test: This code applies specifically to an abnormal oral glucose tolerance test (OGTT) result. It is not a direct code for an abnormal HbA1c, but may be used by coders when a clinician documents an abnormal glucose-related investigation result without a confirmed diagnosis.

• R73.9 – Hyperglycaemia, unspecified: This may be the more appropriate code when a clinician documents hyperglycaemia or an abnormal glucose finding (including HbA1c) without a definitive diagnosis, in line with NHS coding guidance.

Once a diagnosis is confirmed, more specific codes are applied:

• E11 – Type 2 diabetes mellitus (the most common category in adults)

• E10 – Type 1 diabetes mellitus

• E13 – Other specified diabetes mellitus (including secondary or drug-induced diabetes)

• O24 – Diabetes mellitus in pregnancy (including gestational diabetes)

Clinicians and coders should refer to the WHO ICD-10 category notes for R73 and the NHS England National Clinical Coding Standards to ensure accurate and consistent coding. Accurate coding supports appropriate care pathways, commissioning data, and public health surveillance.

How Abnormal HbA1c Levels Are Interpreted Under NHS Guidelines

NICE NG28 recommends a single HbA1c of 48 mmol/mol or above to diagnose type 2 diabetes in symptomatic patients, with a confirmatory repeat test required in asymptomatic individuals.

NHS and NICE guidelines provide clear frameworks for interpreting HbA1c results, though clinical context always informs the final interpretation. NICE guideline NG28 (Type 2 diabetes in adults: management) and NICE guideline PH38 (Type 2 diabetes: prevention in people at high risk) both outline how HbA1c values should guide clinical decision-making. The WHO 2011 guidance on the use of HbA1c in the diagnosis of diabetes mellitus also underpins UK diagnostic thresholds.

For diagnostic purposes, a single HbA1c of 48 mmol/mol or above is sufficient to diagnose type 2 diabetes in a symptomatic individual. In asymptomatic patients, a repeat confirmatory test should be arranged promptly — ideally within approximately two weeks — to reduce the risk of misclassification. It is important to note that HbA1c is not appropriate as a diagnostic tool in certain circumstances, including:

• Suspected type 1 diabetes

• Pregnancy or within three months postpartum (see NICE NG3)

• Haemolytic anaemia, haemoglobinopathies (e.g., sickle cell disease, thalassaemia), or recent blood transfusion

• Conditions affecting red blood cell lifespan or haemoglobin structure

• Rapid-onset hyperglycaemia

In these situations, plasma glucose measurements (fasting or random) are preferred. Severe renal impairment may also affect HbA1c reliability due to both altered red cell lifespan and assay interference (see below), and glucose-based testing should be considered when results appear discordant.

For monitoring purposes in people with established diabetes, NICE recommends individualised HbA1c targets. A target of 48 mmol/mol is generally recommended for most adults with type 2 diabetes managed with lifestyle measures or metformin alone, while 53 mmol/mol may be appropriate for those on medications that carry a hypoglycaemia risk. Targets should be agreed collaboratively with the patient, taking into account comorbidities, frailty, and personal preferences.

In the non-diabetic hyperglycaemia (NDH) range (42–47 mmol/mol), NHS guidance recommends referral to a structured diabetes prevention programme, such as the NHS Diabetes Prevention Programme (NHS DPP), alongside lifestyle advice on diet, physical activity, and weight management.

Clinical Conditions Associated With Abnormal HbA1c Findings

Iron deficiency anaemia, haemoglobinopathies, CKD, and medicines such as corticosteroids can cause falsely elevated or falsely low HbA1c results, requiring clinical correlation before diagnosis.

While an elevated HbA1c most commonly reflects impaired glucose regulation or established diabetes, a range of other clinical conditions can produce abnormal results — either falsely elevated or falsely low — and clinicians must interpret findings in the context of the individual patient's medical history.

Conditions that may cause a falsely elevated HbA1c include:

• Iron deficiency anaemia (reduced red cell turnover prolongs haemoglobin exposure to glucose)

• Vitamin B12 or folate deficiency

• Splenectomy (increased red cell lifespan)

• Certain assay interferences in chronic kidney disease (CKD)

Conditions that may cause a falsely low HbA1c include:

• Haemolytic anaemia (shortened red cell lifespan)

• Haemoglobinopathies such as sickle cell disease or thalassaemia

• Recent blood transfusion

• Erythropoietin therapy

• CKD with reduced red cell survival (note that CKD can affect HbA1c in both directions depending on the mechanism and assay used; when results appear discordant with clinical findings, correlation with glucose profiles or fructosamine measurement is advisable)

Beyond analytical interference, a genuinely elevated HbA1c may be associated with a broader spectrum of metabolic and endocrine conditions. Polycystic ovary syndrome (PCOS), Cushing's syndrome, acromegaly, and chronic pancreatitis can all impair insulin secretion or action, leading to elevated HbA1c values. Certain medicines — including corticosteroids, antipsychotics (particularly olanzapine and clozapine), and some immunosuppressants — are also recognised causes of secondary hyperglycaemia. Patients and clinicians who suspect a medicine may be contributing to an abnormal HbA1c or causing other side effects are encouraged to report this via the MHRA Yellow Card scheme (available at yellowcard.mhra.gov.uk).

It is also important to recognise the cardiovascular implications of abnormal HbA1c. Even within the NDH range (42–47 mmol/mol), there is evidence of increased cardiovascular risk. Clinicians should assess overall cardiometabolic risk using a validated tool such as QRISK, and manage lipid profiles, blood pressure, and renal function as part of a holistic metabolic review, in line with current NICE guidance on cardiovascular risk assessment and lipid management.

Next Steps After Receiving an Abnormal HbA1c Result

An HbA1c of 42–47 mmol/mol warrants referral to the NHS Diabetes Prevention Programme and lifestyle intervention; 48 mmol/mol or above requires prompt confirmatory testing and initiation of structured diabetes management per NICE NG28.

Receiving an abnormal HbA1c result can be understandably concerning for patients, but it is important to understand that a single result does not always confirm a diagnosis and that there are clear, evidence-based steps to follow. The appropriate next steps depend on the level of the result and the clinical context.

For results in the non-diabetic hyperglycaemia (NDH) range (42–47 mmol/mol), the primary focus is on lifestyle intervention. Patients should be:

• Offered referral to the NHS Diabetes Prevention Programme (NHS DPP) — a free, evidence-based structured education programme available to adults in England with an HbA1c of 42–47 mmol/mol

• Advised on dietary changes, including reducing refined carbohydrates and increasing fibre intake

• Encouraged to achieve and maintain a healthy weight, with a target of at least 5–10% body weight reduction where overweight

• Supported to increase physical activity to at least 150 minutes of moderate-intensity exercise per week

• Offered a repeat HbA1c in 12 months, or sooner if symptoms develop

For results at or above 48 mmol/mol, a confirmatory repeat HbA1c should be arranged promptly — ideally within approximately two weeks — unless the patient is symptomatic, in which case a diagnosis can be made on a single result. Once type 2 diabetes is confirmed, NICE NG28 recommends initiating a structured education programme such as DESMOND (Diabetes Education and Self-Management for Ongoing and Newly Diagnosed), alongside dietary advice and pharmacological therapy. In most people, metformin remains the standard first-line agent; however, NICE now recommends considering an SGLT2 inhibitor (with or without metformin) as first-line therapy in people with established atherosclerotic cardiovascular disease (ASCVD), high cardiovascular risk, or chronic kidney disease, given the evidence of cardiorenal benefit.

Patients should also be informed about the importance of regular monitoring, including annual diabetes reviews covering HbA1c, renal function, blood pressure, cholesterol, foot examination, and retinal screening. Empowering patients with knowledge about their condition is central to effective long-term management.

When to Seek Further Assessment or Specialist Referral

Suspected type 1 diabetes, gestational diabetes, diabetic ketoacidosis, or secondary diabetes all require urgent or specialist referral; DKA is a medical emergency requiring immediate 999 attendance.

Whilst the majority of patients with an abnormal HbA1c can be managed effectively in primary care, there are specific clinical circumstances in which prompt further assessment or referral to a specialist is warranted. Recognising these triggers is essential for patient safety.

Patients should contact their GP promptly or seek urgent review if they experience:

• Symptoms of hyperglycaemia: excessive thirst, frequent urination, unexplained weight loss, or blurred vision

• Symptoms suggestive of diabetic ketoacidosis (DKA): nausea, vomiting, abdominal pain, or rapid breathing — call 999 or attend A&E immediately, as DKA is a medical emergency

• Recurrent or severe hypoglycaemia in those already on glucose-lowering therapy

• Rapidly deteriorating renal function or new proteinuria identified on routine monitoring

Referral to a specialist diabetes team or endocrinologist should be considered in the following situations:

• Suspected type 1 diabetes at any age, particularly where there is rapid onset of symptoms, significant weight loss, or ketonuria — these patients require same-day urgent specialist assessment and prompt insulin initiation (NICE NG17)

• Gestational diabetes or pre-existing diabetes in pregnancy, which requires multidisciplinary management under joint obstetric and diabetes care (NICE NG3)

• Secondary diabetes due to an underlying condition (e.g., Cushing's syndrome, acromegaly, pancreatitis), where the primary condition requires specialist management

• Patients with complex comorbidities or those failing to achieve glycaemic targets despite optimised therapy

• Children and young people with any form of diabetes, who should be managed within paediatric diabetes services (NICE NG18)

The MHRA issues periodic safety updates regarding glucose-lowering medicines, and clinicians should remain alert to any new guidance affecting prescribing decisions. Patients who experience suspected side effects from any medicine should report these via the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk. Ultimately, an abnormal HbA1c is a clinical signal — not a diagnosis in isolation — and a thoughtful, patient-centred approach ensures the best possible outcomes.

Frequently Asked Questions