HbA1c NGSP vs IFCC units is a common source of confusion for patients and clinicians alike. HbA1c — glycated haemoglobin — is a key blood test used to diagnose diabetes and monitor long-term blood glucose control across the NHS. Two measurement standards exist: NGSP, which expresses results as a percentage (%), and IFCC, which uses millimoles per mole (mmol/mol). Since 2011, the NHS has reported HbA1c exclusively in IFCC units, yet NGSP percentages remain widely referenced in older literature and international guidelines. This article explains the difference between the two systems, how to convert between them, and what your result means for your diabetes care.

What Are NGSP and IFCC Units for HbA1c?

NGSP expresses HbA1c as a percentage of total haemoglobin, whilst IFCC expresses it in mmol/mol using a more chemically precise molecular reference point; both measure the same glycated haemoglobin marker.

HbA1c — glycated haemoglobin — is a blood test that reflects your average blood glucose level over the preceding two to three months. It is a cornerstone of diabetes diagnosis and long-term management, and is used widely across the NHS. However, for many years, laboratories around the world measured and reported HbA1c using different standards, which caused considerable confusion for patients and clinicians alike.

The two principal measurement standards are:

• NGSP (National Glycohaemoglobin Standardisation Program): Developed in the United States, this system expresses HbA1c as a percentage (%) of total haemoglobin. It was the dominant standard used in landmark trials such as the DCCT (Diabetes Control and Complications Trial) and UKPDS (UK Prospective Diabetes Study).

• IFCC (International Federation of Clinical Chemistry and Laboratory Medicine): This is a more precise, internationally standardised method that expresses HbA1c in millimoles per mole (mmol/mol). It measures the specific fraction of haemoglobin chemically bonded to glucose at a defined molecular site.

The IFCC method was developed to provide a more chemically exact and reproducible measurement, reducing variability between laboratories across different countries. Both systems measure the same underlying biological marker — the proportion of haemoglobin that has been glycated — but they use different reference points and mathematical scales, which is why the numerical values differ significantly between the two.

It is important to note from the outset that HbA1c is not appropriate for diagnosing diabetes in all situations. NICE and WHO guidance identifies several circumstances where it should not be used for diagnosis — these are covered in detail in a later section. Understanding which unit your result is reported in is essential for interpreting it correctly.

How the Two HbA1c Measurement Standards Differ

IFCC measures glycation at a specific molecular site on haemoglobin, producing lower numerical values than NGSP; for example, 7% NGSP equals approximately 53 mmol/mol IFCC.

The fundamental difference between NGSP and IFCC units lies in what each system measures and how it calibrates its results. The NGSP method was historically aligned to the DCCT assay, which measured total glycated haemoglobin as a percentage of all haemoglobin present. This approach, whilst clinically useful and well-validated through decades of research, was not standardised at a molecular level, meaning slight variations could occur between different laboratory methods.

The IFCC method, by contrast, is anchored to a specific chemical reaction: the glycation of the N-terminal valine of the beta chain of haemoglobin. This makes it more analytically precise and reproducible across international laboratories, as established by the IFCC Working Group on HbA1c Standardisation. Because the IFCC method measures a narrower, more specific fraction of glycated haemoglobin, its numerical values are considerably lower than NGSP percentages for the same blood sample.

For example:

• An NGSP result of 7% corresponds approximately to an IFCC result of 53 mmol/mol

• An NGSP result of 6.5% corresponds approximately to 48 mmol/mol in IFCC units

This numerical discrepancy can be confusing if a patient is unfamiliar with the unit being used. A result of 53 mmol/mol may appear very different from 7%, yet they represent the same level of glycaemic control. The two systems are mathematically related through a validated master equation (published by the IFCC and NGSP), which allows reliable conversion between them. Clinicians and patients should always confirm which unit is being used when reviewing results, particularly when comparing readings from different healthcare settings or countries.

Which HbA1c Unit Does the NHS and NICE Use?

The NHS has reported HbA1c in IFCC units (mmol/mol) since 2011, and all NICE guidelines express diagnostic thresholds and treatment targets in mmol/mol.

The NHS in England, Scotland, Wales, and Northern Ireland transitioned to reporting HbA1c in IFCC units (mmol/mol) in 2011, following a recommendation from a joint working group involving Diabetes UK, the Association for Clinical Biochemistry and Laboratory Medicine (ACB), and other professional bodies. This change aligned UK practice with international laboratory standards and improved consistency across NHS laboratories. Some NHS reports and patient-facing materials may still display the NGSP percentage alongside the mmol/mol value for context, but mmol/mol is the UK standard.

NICE guidelines — including those for type 1 diabetes (NG17), type 2 diabetes (NG28), and diabetes in pregnancy (NG3) — express all HbA1c thresholds and targets in mmol/mol. For instance:

• Diagnosis of type 2 diabetes: HbA1c ≥ 48 mmol/mol on two separate occasions (or once in the presence of symptoms of diabetes)

• Non-diabetic hyperglycaemia (increased risk of developing type 2 diabetes): HbA1c of 42–47 mmol/mol — lifestyle intervention is recommended. This range is sometimes referred to as 'prediabetes' in non-UK materials, but 'non-diabetic hyperglycaemia' is the preferred UK term.

• Target for most adults with type 2 diabetes on medication: 53 mmol/mol or below (NICE NG28)

• Target for type 1 diabetes: Individualised, often 48 mmol/mol where safely achievable (NICE NG17)

Important — pregnancy: HbA1c is not used to diagnose or monitor gestational diabetes mellitus (GDM). In pregnancy, capillary plasma glucose targets are used instead. NICE NG3 provides specific guidance on glycaemic management during pregnancy, including for women with pre-existing diabetes.

Despite the 2011 transition, NGSP percentage values remain widely referenced in older clinical literature, patient education materials, and international guidelines — particularly from the American Diabetes Association (ADA). Patients who have been managing diabetes for many years may still be more familiar with percentage values. NHS clinicians are encouraged to support patients in understanding both systems where necessary, though official NHS communications use mmol/mol as the standard unit.

Converting Between NGSP (%) and IFCC (mmol/mol) Values

Use the formula IFCC (mmol/mol) = (NGSP % × 10.929) − 23.5 to convert between units; Diabetes UK also provides an online conversion calculator.

Converting between NGSP and IFCC values is straightforward using a validated mathematical formula derived from the IFCC–NGSP master equation. The relationship between the two units is linear, and the conversion can be performed reliably in either direction.

To convert NGSP (%) to IFCC (mmol/mol): IFCC (mmol/mol) = (NGSP % × 10.929) − 23.5

To convert IFCC (mmol/mol) to NGSP (%): NGSP (%) = (IFCC mmol/mol + 23.5) ÷ 10.929

For everyday clinical use, the following reference table provides a quick guide to commonly encountered values:

| NGSP (%) | IFCC (mmol/mol) | |---|---| | 5.0% | 31 | | 5.7% | 39 | | 6.0% | 42 | | 6.5% | 48 | | 7.0% | 53 | | 7.5% | 58 | | 8.0% | 64 | | 9.0% | 75 |

These conversions are based on the IFCC–NGSP master equation, which is available via the NGSP website and the IFCC Working Group on HbA1c Standardisation. Diabetes UK also provides an online conversion calculator to assist patients and clinicians.

It is important to note that whilst conversion is mathematically precise, small rounding differences may occasionally appear between laboratory systems. Patients should not attempt to self-adjust their treatment based on converted values alone — any concerns about HbA1c results should always be discussed with a GP or diabetes specialist nurse. The conversion tool is best used as an aid to understanding, not as a substitute for clinical interpretation.

How HbA1c Results Affect Your Diabetes Diagnosis and Targets

An HbA1c of 48 mmol/mol or above confirms a diagnosis of type 2 diabetes; 42–47 mmol/mol indicates non-diabetic hyperglycaemia requiring lifestyle intervention.

HbA1c plays a central role in both the diagnosis of diabetes and the ongoing monitoring of glycaemic control. Understanding your result — and the unit it is expressed in — is therefore clinically important.

For diagnosis, NICE (NG28) and the World Health Organisation (WHO 2011 guidance on HbA1c for the diagnosis of diabetes) define the following thresholds using IFCC units:

• Below 42 mmol/mol: Normal range — diabetes is unlikely

• 42–47 mmol/mol: Non-diabetic hyperglycaemia — increased risk of developing type 2 diabetes; lifestyle intervention is recommended

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

When HbA1c should NOT be used for diagnosis

HbA1c is not appropriate for diagnosing diabetes in the following situations, as results may be unreliable or misleading:

• Pregnancy (including suspected gestational diabetes — use plasma glucose testing per NICE NG3)

• Children and young people (plasma glucose testing is preferred)

• Suspected type 1 diabetes or acute onset of hyperglycaemic symptoms (diagnosis should be based on plasma glucose; urgent clinical assessment is required)

• Acute pancreatic damage or pancreatic surgery

• Conditions affecting red blood cell turnover, including haemolytic anaemia, haemoglobinopathies (e.g., sickle cell disease, thalassaemia), recent blood transfusion, iron deficiency anaemia, and advanced chronic kidney disease or erythropoietin therapy

In these situations, a fasting plasma glucose (FPG) or oral glucose tolerance test (OGTT) should be used instead. Your GP or diabetes team can advise on the most appropriate test for your circumstances.

For ongoing management, treatment targets are individualised based on the type of diabetes, medications used, age, comorbidities, and risk of hypoglycaemia. General NICE-aligned targets include:

• Type 2 diabetes managed by lifestyle or metformin alone: 48 mmol/mol (NICE NG28)

• Type 2 diabetes on drugs that carry a risk of hypoglycaemia (e.g., sulphonylureas or insulin): 53 mmol/mol (NICE NG28)

• Type 1 diabetes: 48 mmol/mol where safely achievable (NICE NG17)

HbA1c also does not capture glucose variability or episodes of hypoglycaemia, which is why continuous glucose monitoring (CGM) is increasingly used alongside it in type 1 diabetes management.

If you are taking medicines for diabetes and think you may be experiencing a side effect, you can report this to the Medicines and Healthcare products Regulatory Agency (MHRA) via the Yellow Card scheme at yellowcard.mhra.gov.uk.

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Talking to Your GP or Diabetes Team About Your HbA1c Result

Always confirm whether your result is in mmol/mol or percentage, and contact your GP promptly if your HbA1c has risen significantly or you have symptoms of poorly controlled blood glucose.

Receiving an HbA1c result can raise many questions, particularly if you are unsure which unit is being used or how the number relates to your diabetes management. Open communication with your GP, practice nurse, or diabetes specialist team is the most effective way to ensure you understand your result and what action, if any, is needed.

When you receive your HbA1c result, it is helpful to ask:

• Is this result in mmol/mol or percentage?

• How does this compare to my previous results?

• Am I within my agreed target range?

• Do I need any changes to my medication, diet, or lifestyle?

You should contact your GP or diabetes team promptly if:

• Your HbA1c has risen significantly since your last test

• You are experiencing symptoms of poorly controlled blood glucose, such as excessive thirst, frequent urination, fatigue, or blurred vision

• You have been told your result is in the non-diabetic hyperglycaemia range (42–47 mmol/mol) and have not yet received lifestyle advice or a follow-up plan

• You are pregnant or planning a pregnancy, as tighter glycaemic targets apply and different monitoring methods are used

Seek urgent medical attention if you develop severe symptoms such as vomiting, rapid breathing, confusion, or feel very unwell alongside high blood glucose readings or ketones in your urine or blood. These may be signs of diabetic ketoacidosis (DKA) or hyperosmolar hyperglycaemic state (HHS), both of which require immediate assessment.

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For patients managing diabetes at home, it is worth keeping a personal record of HbA1c results over time, noting both the value and the unit used. This is particularly useful if you access care across different settings or travel internationally, where NGSP percentage values may still be used. Diabetes UK (www.diabetes.org.uk) and the NHS website (www.nhs.uk) offer reliable, up-to-date patient resources to support self-management and help you interpret your results with confidence.

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