HbA1c – Diagnostic threshold and treatment targets
HbA1c (or glycated haemoglobin) refers to a fraction of haemoglobin that has been modified by the presence of glucose over the lifespan of the red blood cell (around 120 days). Higher average blood glucose concentrations are reflected by higher HbA1c.
HbA1c for the diagnosis of diabetes mellitus
An HbA1c ≥ 6.5% or 48 mmol/mol is consistent with a diagnosis of diabetes mellitus.
Medicare will provide a patient rebate for one diagnostic HbA1c per annum.
In patients without typical symptoms of diabetes mellitus, two positive tests are required to confirm the diagnosis (i.e. in addition to an initial HbA1c result ≥ 6.5%, a repeat HbA1c ≥ 6.5%, a fasting blood glucose > 7.0 mmol/L, or a blood glucose > 11.0 mmol/L two hours after a 75 g glucose load).
HbA1c is not a suitable stand-alone diagnostic test for gestational diabetes or type 1 diabetes mellitus.
HbA1c for monitoring diabetes mellitus
In patients with established diabetes mellitus, treatment targets should be individualised based on a range of factors. A “general” target for most patients is ≤ 7.0% or 53 mmol/mol. Patient groups in whom less stringent targets may be considered include those at higher risk of hypoglycaemia, older patients or those with comorbidities, and the very young. More stringent targets may be used in patients with type 2 diabetes mellitus of shorter duration. Further information is available in this Position statement of the Australian Diabetes Society.
In women with pre-existing diabetes mellitus planning pregnancy, an HbA1c target of < 6.5% is advocated to reduce the risk of fetal malformations, provided that the risk of hypoglycaemia in the individual patient is deemed to be sufficiently low when aiming for this target. Further information is available in this Position Statement of the Australian Diabetes in Pregnancy Society.
Medicare will provide a patient rebate for HbA1C when used in monitoring known diabetes four times per annum. Stating whether the patient has diabetes on the request form will help the patient to avoid receiving a bill.
“At target” does not always mean good glycaemic control
As with any laboratory test, meaningful interpretation can only be made in the overall clinical context. For example, whilst an “at target” HbA1c (e.g. 6.0%) in a patient with diabetes mellitus could indicate excellent glycaemic control, poorly controlled diabetes with frequent, alternating hypoglycaemia and hyperglycaemia over a period of weeks or months could also cause this result.
Causes of clinically misleading HbA1c results
While the most important contributor to HbA1c is blood glucose, several factors can cause increases or decreases in HbA1c which may be clinically misleading:
Measurement issues and identification of haemoglobin variants
The method of measurement for HbA1c at Western Diagnostic Pathology is high-performance liquid chromatography. This method enables us to identify possible haemoglobin variants which may (1) cause misleading HbA1c results; (2) have possible inherent clinical significance; or (3) be of no significance at all. Sometimes we will provide a comment advising that a variant has been detected; haemoglobin electrophoresis may be recommended. If we are concerned about possible analytical interference, we may need to perform additional testing, which may cause a delay in reporting of the result.
Authors:
Dr Johan Conradie FRCPA (HOD)
Dr Melissa Gillett FRACP FRCPA
Dr Michael Page FRCPA
Dr Ranita Siru FRACP (Registrar)
Department of Clinical Biochemistry
Western Diagnostic Pathology
References and further reading
D’Emden MC, Shaw JE, Colman PG et al. The role of HbA1c in the diagnosis of diabetes mellitus in Australia. Med J Aust 2012; 197: 220-221. Available from: https://www.mja.com.au/journal/2012/197/4/role-hba1c-diagnosis-diabetes-mellitus-australia
Wah Cheung N, Conn JJ, d’Emden MC et al. Position statement of the Australian Diabetes Society: individualisation of glycated haemoglobin targets for adults with diabetes mellitus. Med J Aust 2009; 191: 339-344. Available from: https://www.mja.com.au/journal/2009/191/6/position-statement-australian-diabetes-society-individualisation-glycated
