CLINICAL BIOCHEMISTRY

Guidance note # 9

Controlled Document CI-CB-337-GN9 Version 5
Reviewed biennially. Last reviewed Sept 2015. / Prepared by: Dr Sally Brady, Principal Biochemist
Authorised by: Dr Anne Dawnay, Consultant Biochemist
Haemoglobin (Hb)-A1c
HbA1c is one of the glycated haemoglobins / Proteins steadily become ‘glycated’, binding extra sugar from the time they are made to the time they are broken down and removed. HbA (adult haemoglobin) is one such protein and its glycated form is called HbA1. HbA1c is one subtype with added glucose.
HbA1c ‘integrates’ a patient’s glucose level over time / Glycation is a slow irreversible process, with the rate depending on the glucose concentration and half-life of the protein. The HbA1c, as a fraction of all HbA present, represents the average rate of glycation over the life of HbA (about 120 days) and particularly in the previous 6-8 weeks. This integrated view of glucose control is useful in the long term management of people with diabetes.
HbA1c is reported in mmol/mol / HbA1c is reported in units of mmol/mol HbA as well as % units, as part of worldwide standardisation. Guide unit conversion table:
New Units mmol/mol (IFCC) / 20 / 42 / 48 / 53 / 59 / 64 / 75
Old Units % (DCCT) / 4.0 / 6.0 / 6.5 / 7.0 / 7.5 / 8.0 / 9.0
In some cases the HbA1c cannot be properly interpreted / HbA1c will be decreased if red cell life is shortened, eg haemolytic and sickle cell anaemias, and increased if red cell life is lengthened, eg iron-deficiency anaemia and after splenectomy. Results are uninterpretable for at least 3 months following blood transfusion.
HbA1c should not be requested in patients with a known haemoglobinopathy / HbA1c cannot be reliably identified when substantial amounts of non-HbA haemoglobin are present and should not be requested in patients with a haemoglobinopathy. The HbA1c method used at UCLH usually detects such patients and the sample will be reported as unsuitable. Such patients may require haemoglobin typing (by Haematology). Diabetes control may be assessed alternatively using serum fructosamine (glycated serum protein) that reflects glycaemic control over the previous few weeks. Fructosamine cannot be used for diagnosis of diabetes.
HbA1c as a diagnostic test for diabetes / HbA1c cannot be used to diagnose Type 1 or gestational diabetes. The WHO and Diabetes UK only recommend HbA1c for diagnosis in the absence of genetic, haematological and illness-related factors (see above). An HbA1c result ≥48 mmol/mol is diagnostic in symptomatic patients but must be repeated to confirm in asymptomatic patients. A value <48mmol/mol does not exclude diabetes using glucose tests. Alternative tests include fasting glucose and OGTT, but diagnosis should not be made using a combination of tests.
Testing frequency depends on diabetic control / Good diabetic control: Check at annual review
Poor diabetic control: Check every 3 months
Very poor diabetic control: Check every month (as encouragement)
Pregnant diabetic control: Check every month

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