What tests can be done to find out if I have diabetes?

There are a number of tests that may be done to find out if you have diabetes. Laboratories in New Zealand operate under strict quality control. This means that the tests you have done at the laboratory are likely to be very reliable and accurate.

Screening for diabetes in people without symptoms

As part of Cardiovascular risk assessment (CVRA) that is done by GPs, all adults should have an HbA1c performed, in men from 45 years of age and women from 55 years of age with the frequency of follow up screening based on the risk assessment. These CVRA are done from 30 years in men 40 years in women if you are Māori, Pacific or Indo-Asian ethnicity.

Two abnormal tests (HbA1c or fasting) are required to confirm the diagnosis either on same day or subsequent test without delay.

If you have ‘prediabetes’ (HbA1c 41 – 49 mmol/mol or fasting glucose 6.1 – 6.9 mmol/L or 2 hour glucose on GTT 7.8 – 11 mmol/L) should have an HbA1c measured annually to determine if you have progressed to type 2 diabetes.

Women with a history of gestational diabetes should have an HbA1c measured annually to determine if they have progressed to type 2 diabetes.

If you have two of more risk factors for diabetes (as below) you should have an HbA1c performed at least every 3 years:

  • Non-European ethnicity

  • Previous ‘prediabetes’ or Type 2 Diabetes in remission

  • First degree relative developing type 2 diabetes at < 40 years of age

  • Known polycystic ovarian syndrome, acanthosis nigricans (a skin condition - thicker and darker patches or streaks, usually in skin creases and folds), high blood pressure, high cholesterol (blood fats) etc.

  • History of cardiovascular disease

  • Current long-term steroids or antipsychotic therapy

  • Post-transplant

  • Obesity

  • BMI over 30 kg/m2 or over 27 kg/m2 if Indo-Asian ethnicity

  • Waist circumference over 94 cm in males or over 80 cm in women if large muscle mass

Oral Glucose Tolerance Test

If your HbA1c is unreliable (as below) a fasting glucose or a 75 g oral glucose tolerance test should be performed:

  • If you have Sickle cell anaemia or Thalassemia, or other blood conditions (check with your GP)

  • Altered blood red cell turnover e.g. bleeding, haemolysis (breakdown or destruction of red blood cells), significant iron deficiency etc.

  • After a blood transfusion

  • Second and third trimesters of pregnancy (between 3-9 months)

If your fasting glucose is ≥ 7 mmol/L or 2 hour glucose is ≥ 11.1 mmol/L you have diabetes.

If your fasting glucose is < 7 mmol/L and 2 hour glucose is ≤ 11 mmol/L then repeat an HbA1c and fasting glucose in 3 months.

  • If the repeat HbA1c is ≥ 50 mmol/L or fasting glucose is ≥ 7 mmol/L you have diabetes

  • If the repeat HbA1c is 41 - 49 mmol/mol and fasting glucose 6 - 6.9 mmol/L you have ‘prediabetes’.

Screening for diabetes in people who have symptoms

If you have symptoms of hyperglycaemia/insulin deficiency (see below) you should have an HbA1c and fasting or random glucose level performed:

  • Passing urine frequently

  • Increased thirst

  • Sudden weight loss

  • Recurrent fungal, skin or infections of the urinary tract (UTIs)

You will be diagnosed if either the HbA1c is ≥ 50 mmol/mol, fasting glucose ≥ 7 mmol/L or random glucose ≥ 11.1 mmol/L.

Oral Glucose Tolerance Test (OGTT) to diagnose diabetes in pregnancy

An oral glucose tolerance test is a test where you go to the laboratory before eating anything in the morning. A blood glucose level is taken. Then you will be asked to drink a glass of fluid containing 75 grams of glucose (this is a lot!). You stay in the laboratory and another blood glucose level is taken at one hour and two hours after the drink. These blood glucose levels tell you how well your body uses and stores the glucose you have taken in the drink.

If you are not pregnant and your blood glucose two hours after an OGTT is 11.1mmol/L or more you have diabetes.

If you are having a glucose tolerance test to see whether you have diabetes of pregnancy (gestational diabetes), and your blood glucose two hours after the glucose drink is 9mmol/L or more, you have diabetes of pregnancy (gestational diabetes).

Screening of pregnant women for diabetes:

All pregnant women without known diabetes should have an HbA1c performed < 20 weeks gestation to detect any undiagnosed pre-gestational diabetes or prediabetes.

  • HbA1c ≥ 50 mmol/mol – you will be referred to specialist Diabetes in Pregnancy (DiP) team and obstetric service and treat as pre-gestational diabetes as above

  • HbA1c 41 – 49 mmol/mol – you will be referred to specialist DiP team and obstetric service (DHB dependent)

  • HbA1c ≤ 40 mmol/mol (no diabetes) – screen between 24 – 28 weeks gestation as per below:

    • All pregnant women without diabetes should have either a 50 g glucose challenge test (if previous HbA1c ≤ 40 mmol/mol ) or a 75 g glucose tolerance test (if previous HbA1c 41 -49 mmol/mol) between 24-28 weeks gestation to find out whether they have gestational diabetes (GDM).

50g glucose challenge test: 

  • Blood glucose level (BGL) ≥ 11.1 mmol/L – you will be referred to specialist DiP team and obstetric service

  • BGL 7.8 – 11 mmol/L – take a 75 g GTT as soon as possible

  • BGL ≤ 7.7 mmol/L – no further screening for Gestational Diabetes Mellitus (GDM) is required unless you have some symptoms of diabetes

Glucose tolerance test results:

  • Fasting BGL ≥ 5.5 mmol/L and/or 2 hour BGL ≥ 9 mmol/L – refer to specialist DiP team and obstetrics service

  • Fasting BGL ≤ 5.4 mmol/L and 2 hour BGL ≤ 8.9 mmol/L - no further screening for GDM is required unless clinically warranted

Women who have had GDM have up to a 50% risk of developing type 2 diabetes within 5 years after their baby’s birth. Consequently, women who have had GDM should have an HbA1c measured three months post-delivery and then annually to ensure they have not developed type 2 diabetes. As per prediabetes, these women should also be offered lifestyle management advice with or without the tablet metformin to lose weight to 90% of their pre-pregnancy weight and to reduce their likelihood of developing Type 2 diabetes.

Random blood glucose level

If you have the symptoms of diabetes, you may be asked to go to the laboratory for a random blood glucose test. This is a blood glucose level taken at any time of the day after you have eaten at any time (i.e. not in a fasting state). If this level is 11.1mmol/L or more, you have diabetes.

Islet Cell Antibody tests

Many people with Type 1 diabetes have antibodies in their blood that show the autoimmune process that results in Type 1 diabetes. If you have a direct relative (parent, sibling or child) with Type 1 diabetes, you can have this test done. If you have these antibodies it’s extremely likely that you will develop Type 1 diabetes.