Diagnostic Use
Haemoglobin A1c is an index of metabolic control in patients with diabetes mellitus.
The result correlates best with the mean level of blood glucose during the previous 8 weeks, with the mean blood glucose during the last 30 days prior to testing contributing about 50% of the final result.
HbA1c is formed continuously during the lifespan of the RBC. The HbA1c level depends on (a) the average glucose level , (b) the mean RBC age, and (c) other factors which are not well understood but which are constant for an individual.
Interpretation
Unexpectedly low HbA1c occurs when there is increased RBC turnover (reduced mean RBC age):
– Ongoing blood loss, with replacement, e.g. GI bleeding, menorrhagia, venesection
– Haemolysis
– Unstable haemoglobin (e.g. some haemoglobinopathies, thalassaemias)
– Renal failure (red cell lifespan shortened up to 40%)
– Starting iron, B12 or folate in deficient patients or starting erythropoietin in renal failure patients
– Recent transfusion
– Pregnancy
– HIV-infected patients on antiretrovirals
Unexpectedly high HbA1c occurs when there is increased mean red cell age:
– Nutritional iron deficiency
– Hyposplenism
– Bone marrow failure
These influences mean that interpretation of the HbA1c level should always be in the context of other clinical and laboratory data.
Analytical Issues
Some variant haemoglobins produce erroneous HbA1c results which are often method specific. The possibility of an analytical artefact should be considered when:
– Widely varying results between different laboratories (which may be using different methods)
– Extreme HbA1c results (e.g. over 150 mmol/mol, or below 20 mmol/mol)
– Lack of apparent correlation between HbA1c and repeated glucose results
– Known haemoglobinopathy or presence of significant amounts of HbF.
Reference Intervals
When performed for diagnosis or cardiovascular disease risk screening:
|
mmol/mol
(IFCC units)
|
|
40 or less
|
Virtually excludes diabetes. No need to repeat until next scheduled CVD risk assessment |
|
41 – 49
|
Suggests abnormal glucose tolerance. Recommend diet/lifestyle changes and assess/manage all CVD risk factors. Repeat annually unless symptomatic in interim. |
| 50 or greater |
Supports a diagnosis of diabetes but confirmation is required in asymptomatic patients. |
Glucose-based diagnostic criteria should always be used in cases of haemoglobinopathy or increased red cell turnover.
In those with confirmed diabetes
|
mmol/mol
(IFCC units)
|
|
| less than 50 |
Excellent control; increased risk of hypoglycaemia if on insulin/sulphonylureas |
| 50 – 54 |
Very good control; some risk of hypoglycaemia if on insulin/sulphonylureas |
| 55 – 64 |
May be appropriate and acceptable in many individuals but higher than ideal. Microvascular complication risk increases markedly above 55 mmol/mol |
| 65 – 79 |
Suboptimal glycaemic control. Consider more intensive treatment. Microvascular complication risk increases markedly above 55 mmol/mol |
| 80 – 99 |
Poor glycaemic control. More intensive treatment recommended. Microvascular complication risk increases markedly above 55 mmol/mol |
| 100 or more |
Very poor glycaemic control. Warrants immediate action |
In pregnancy
| mmol/mol
(IFCC units) |
| <41 |
Unlikely to have pre-existing glucose tolerance/diabetes, but can develop gestational diabetes. Follow local guidelines. |
| 41 – 49 |
May reflect glucose intolerance. Follow local guidelines, or offer 75g oGTT at 24-28 weeks. |
| 50 or greater |
Levels consistent with pre-existing diabetes. Refer to local diabetes in pregnancy service. |
Glycated haemoglobin measurements may be misleading in cases of haemoglobinopathy or increased red cell turnover.
Units conversion:
NGSP units (%) = 0.09148 x IFCC units + 2.15
IFCC units (mmol/mol) = 10.93 x [NGSP units – 2.15]
| % |
mmol/mol |
| 4.0 |
20 |
| 4.5 |
26 |
| 5.0 |
31 |
| 5.5 |
37 |
| 6.0 |
42 |
| 6.5 |
48 |
| 7.0 |
53 |
| 7.5 |
58 |
| 8.0 |
64 |
| 8.5 |
69 |
| 9.0 |
75 |
| 9.5 |
80 |
| 10.0 |
86 |
| 10.5 |
91 |
| 11.0 |
97 |
| 11.5 |
102 |
| 12.0 |
108 |
| 12.5 |
113 |
| 13.0 |
119 |
Test Method
Principle: Capillary electrophoresis
Assay: Sebia
Analyser: Octa Capillarys 3