HbA1c

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.