Diagnostic Use
Haemoglobin A1c is useful for diagnosis and monitoring of diabetes mellitus in most patients, though there are a few exceptions described under ‘Limitations/Interference’ below.
HbA in red blood cells can be gradually glycated by circulating glucose, thus hyperglycaemia increases HbA1c. 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. 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.
Interpretation
Results are reported with an interpretive comment to aid diagnosis and monitoring.
Refer to the Auckland Region HealthPathways for local diagnostic guidelines. Additionally, there are entries with advice on management in specific clinical scenarios.
Results may be unreliable when the patient has had a recent blood transfusion, increased/decreased red blood cell turnover or Hb variant. See ‘Limitations/Interference’ below for more details.
Reference Intervals
The expected/acceptable HbA1c value for a patient can vary based on factors such as age, ethnicity and health conditions. See the report comment for specific interpretation advice.
In general:
- Below 39 mmol/mol is normal.
- 39-41 mmol/mol in a young person may indicate increased risk of progression to prediabetes/diabetes, particularly if they have risk factors, including Maori, Pacific and/or Indo-Asian ethnicity.
- 42-47 mmol/mol is indicative of pre-diabetes (unless the patient already has a diabetes diagnosis).
- 48-52 mmol/mol could be consistent with diabetes mellitus, but confirmation is advised (e.g. repeat HbA1c).
- Above 52 mmol/mol is consistent with diabetes mellitus.
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
Limitations / Interference
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.
Uncertainty of Measurement
6%