Haemoglobin A1c (HbA1c) is a measure of glycated haemoglobin and provides an assessment of glycaemic control over a prolonged time period. It has a well-established role in the monitoring of Diabetes Mellitus (DM) and also, more recently in diagnosis. Haemoglobin undergoes glycation at a rate that is directly proportional to the glucose concentration in the blood and forms a stable glycated product (HbA1c), this then gives an indication of the glycaemic control over the previous 6-8 weeks.

Monitoring Diabetes Mellitus

Measurement of HbA1c should be carried out in individuals with diabetes to support treatment decisions and optimise glycaemic control. This should be no more frequently than that described in the section below on when to repeat a test.

Diagnosis of Type 2 Diabetes Mellitus (T2DM)

Local policy should be followed in the diagnosis of Type 2 DM and should not be based on a single test result. Results should be interpreted in conjunction with the patient’s symptoms. The first line test for the diagnosis of T2DM may be a fasting glucose. A fasting blood glucose ≥7.0 mmol/L and classical osmotic symptoms is diagnostic of T2DM. If the fasting blood glucose is 6.1-7.0 mmol/L or ≥7.0 mmol/L and the patient is asymptomatic, this can be followed up with HbA1c or a repeat fasting glucose. At least 1 additional HbA1c or glucose result within the diabetic range should be obtained within 2 weeks of the initial test if the patient is asymptomatic. An HbA1c of ≥48mmol/mol (6.5%) is recommended as the cut off point for diagnosing diabetes. “Impaired” or “Pre-diabetes” is classified as having and HbA1c between 42 – 47 mmol/mol, and normal results are 41 mmol/mol HbA1c and below.

Other considerations

• Follow up of woman with Gestational Diabetes
• Assessment of those deemed to be ‘at high risk’ of Type 2 diabetes as identified by using an online risk stratification tool e.g. Diabetes UK – Know Your Risk of Type 2 diabetes.

 Do not request HbA1c in the initial diagnosis of Type 1 diabetes, as a diagnostic test in children or pregnant women, if symptoms are of a short duration or if the patient is acutely unwell.

 Diagnosis of Type 2 diabetes should not be made on the basis of a single abnormal plasma glucose or HbA1c value in an asymptomatic patient.

 HbA1c is not recommended for screening patients <45 years old with normal weight and no risk factors.

 HbA1c should not be measured in patients with known haemoglobinopathies which interfere with the methodology, or in patients with anaemia. Conditions which alter the red blood cell lifespan (e.g rheumatoid arthritis, spenomegaly, some drugs) can also result in inaccurate HbA1c results.

 HbA1c should not be used to monitor blood glucose control in the 2^nd and 3^rd trimesters of pregnancy

RCPath, ACB and IBMS guidance on minimum retest intervals in Pathology states that HbA1c should be measured at 3-6 monthly intervals (tailored to individual needs) until the blood glucose concentration is stable on unchanging therapy.A measurement made at an interval of less than 3 months can be used as an indicator of direction of change, rather than as a new steady state. HbA1c should be measured at 6-monthly intervals once the blood glucose concentration and therapy are stable in an individual with known diabetes.

More frequent monitoring may need to be carried out in those with diabetes who are:

• Pregnant or trying to conceive (up to monthly measurements)
• Have recently changed their diabetes medication
• Have poorly controlled diabetes

Annual HbA1c/fasting glucose is recommended for those at ‘high risk’ of diabetes e.g. prediabetes/ GDM.

1. National Institute for Health and Care Excellence. Type 2 diabetes in adults: management [Internet]. [London]: NICE; 2015 [updated 2022 Jun; cited 2022 Oct 04]. (NICE guideline [NG28]). Available from: https://www.nice.org.uk/guidance/ng28
2. National Institute for Health and Care Excellence. Diabetes in pregnancy: management from preconception to the postnatal period [Internet]. [London]: NICE; 2015 [updated 2022 Dec; cited 2022 Oct 04]. (NICE guideline [NG3]). Available from: https://www.nice.org.uk/guidance/ng3
3. Lang T., Croal B. National minimum retesting intervals in pathology [Internet]. 2nd ed. The Royal College of Pathologists; 2021 [cited 2022 Oct 04]. Available from: https://www.rcpath.org/resourceLibrary/g147-minimum-retesting-intervals-in-pathology.html

To be done

To be done