Diagnostic Use
Notice of Updated HbA1c Dx threshold from 1 7 26 – guidance for health sector – HNZ – rec 15 4 26
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 individuial.
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 30 mmol/mol)
– Lack of apparent correlation between HbA1c and repeated glucose results
– Known haemoglobinopathy or presence of significant amounts of HbF. Please check with Chemical Pathologist at Biochemistry laboratory, on ext 58519 re: if the analytical method used is affected by your patient’s haemoglobinopathy.
Other than mean glucose/time in range data from continuous glucose monitoring, alternative markers like Fructosamine (serum) or Glycated Albumin (serum/EDTA plasma) can be used for DM monitoring – recommend discussion with Chemical Pathologist before blood collection.
Reference Intervals
| HbA1c (non-pregnant individuals)
(mmol/mol) |
Comment (for more details – see “Notice of Updated HbA1c diagnostic threshold from 1st July 2026 – guidance for health sector, 24th March 2026 – Health New Zealand” pdf document under “Diagnostic Use” section) |
| <30 |
May be normal in young healthy subjects. For known diabetes, beware of significant hypoglycaemia risk and adjust therapy accordingly. Other cases include increased red cell turnover e.g. haemolysis, recent blood transfusion and Hb variants if discordant from other glycaemic parameters. Contact Chemical Pathologist if alternative biomarker is required to diagnose or monitor diabetes. |
| ≥30-38 |
Excludes diabetes if used for screening. For known diabetes, consider reduction of glucose-lowering therapies especially if treated with insulin and sulfonylureas |
| ≥39-41 |
Excludes diabetes if ≥25 years of age. However, can indicate high risk of progression if <25 years, particularly if of Māori, Pacific or Indo-Asian ethnicity, or high clinical suspicion. Address lifestyle factors and repeat test in 6 months. For known diabetes, consider reduction of glucose-lowering therapies especially if treated with insulin and sulfonylureas |
| ≥42-47 |
If screening, indicates prediabetes. Suggest diet, lifestyle and any other intervention as appropriate and repeat in 6-12 months. For known diabetes, it is on target for all ages but consider hypoglycaemia risk if on insulin and/or sulfonylurea |
| ≥48-52 |
If not known to have diabetes, repeat test as soon as is practical. Repeat level ≥48mmol/mol confirms the diagnosis (or alternatively fasting glucose ≥7mmol/L or random glucose ≥11mmol/L if symptomatic).
For known diabetes, it is on target in most adults, but consider hypoglycaemia risk if on insulin and/or sulfonylureas. Escalation of care recommended if young, pregnant or considering pregnancy, or significant burden of microvascular complications aiming for HbA1c <48mmol/mol if hypoglycaemia risk low |
| ≥53-64 |
If screening, diabetes is confirmed and no repeat is needed. For known diabetes, escalation of care aiming for <53mmol/mol in most adults, or <48mmol/mol if young, pregnant or considering pregnancy, or significant burden of microvascular complications if low risk of hypoglycaemia. However, this HbA1c may be appropriate if life expectancy is limited by non-diabetes comorbidities and/or high risk of hypoglycaemia |
| ≥65-89 |
Suboptimal to poor control of diabetes. Consider additional glucose lowering therapy |
| ≥90 |
Very poor control of diabetes. Unless contraindicated, consider additional glucose lowering therapy including insulin if not already instituted |
Unit conversion:
NGSP units (%) = 0.09148 x IFCC units (mmol/mol) + 2.15
IFCC units (mmol/mol) = 10.93 x [NGSP units – 2.15]
| HbA1c (mmol/mol) – Pregnancy <20 weeks gestation, with no known diabetes |
Comment – details refer to local health pathway/guidelines : |
| ≤ 40 |
Screen between 24-28 weeks gestation, use :
– 50g Glucose Challenge Test (GCT) (for individuals without high risk factors)
– 75g Oral Glucose Tolerance test (OGTT) (for individuals with high risk factors e.g. BMI >30 kg/m2 ; Age >30 ; PCOS ; Chronic hypertension ; Steroid or antipsychotic medication ; Family history of diabetes in a first degree relative ; Large for gestational age; Non-white ethnicity; Previous GDM/Macrosomia/Perinatal loss/Premature birth) |
| 41-49 |
Screen between 24-28 weeks gestation – use 75 g Oral Glucose Tolerance (OGTT) test |
| ≥ 50 |
Consistent with Pre-gestational Diabetes. Refer to Diabetes in Pregnancy specialist service |
If had bariatric surgery, then GCT or OGTT test are not appropriate – refer to local guideline.
Women who have had GDM should have an HbA1c measured 3 months post-delivery and then annually. Also suggest preconception HbA1c with GP prior to next pregnancy.
Test Method
Principle: HPLC - Boronate affinity
Analyser: Trinity Biotech Premier Hb9210 (Primus)