Blood tests for diabetes

The main test used for screening, diagnosis and monitoring of diabetes is HbA1C, but sometimes other tests are also used.

Key points about diabetes blood tests

  1. Adults over a certain age should be screened at least every 3–5 years for diabetes. Pregnant women and some children should also be screened.
  2. The HbA1c test measures the amount of glucose in your blood over the previous 3 months and is used to screen for diabetes. 
  3. A high HbA1c result usually means you have diabetes. 
  4. Other tests can also be used, eg, fasting glucose test or a 2-hour glucose tolerance test if you are pregnant.
  5. If you have been diagnosed with diabetes, the HbA1c test and/or blood glucose tests are also used to monitor your diabetes.

On this page you can find the following information:

Why are blood tests done to screen for diabetes?

Many people who have diabetes don't feel unwell in the early stages. By the time you get symptoms of diabetes, there may already be damage to important parts of your body. Treatment of diabetes can slow down further complications but can’t usually repair this damage. This is why your healthcare provider might test you for diabetes even if you feel well. 

Testing people for a condition like diabetes when they do not have any symptoms is called screening. Studies have been done to work out who is more likely to have diabetes so they can be screened. 

Diabetes is more common in:

  • Māori, Pasifika and Indo-Asian people
  • older people
  • people who are overweight
  • people who have whānau/family members with diabetes.

These people should have regular screening tests for diabetes. 

Who should be screened for diabetes?

In Aotearoa New Zealand, screening for type 2 diabetes is part of the routine testing for heart and blood vessel (cardiovascular) disease. This is usually done at least every 3–5 years in adults over a certain age. The age to start screening depends on your ethnicity, family history and other medical history. The checks include:

Read more about cardiovascular risk assessments. 

What age does screening for diabetes start?

Group

Men

Women

People without symptoms or known risk factors

Age 45 years Age 55 years

Māori, Pasifika and Indo-Asian people

Age 30 years Age 40 years

People with other known risk factors, including:

  • having a close family member, such as a parent, brother or sister with diabetes
  • being a smoker (or having quit only in the last 12 months)
  • having high blood pressure
  • having had diabetes in any pregnancy
  • being overweight (use our BMI calculator)
Age 35 years Age 45 years

People with severe mental illness

Age 25 years Age 25 years

Why are some children screened for diabetes?

Diabetes can also occur in children. Most children with diabetes have type 1 diabetes. In type 1 diabetes the pancreas cannot produce enough insulin. However, children who become overweight can also develop type 2 diabetes. Unfortunately, this is becoming more common. For this reason, obese children and young people should have a screening blood test for diabetes, especially if there are other people in the whānau/family who have diabetes. 

Why do pregnant women need to be screened for diabetes?

During pregnancy, some women develop a type of diabetes known as gestational diabetes. Gestational diabetes happens to about 4–8% of all pregnant women. It can lead to problems for both mother and baby if it is not treated. This is why all pregnant women should be screened for diabetes. Read more about gestational diabetes.

How is diabetes diagnosed?

Most of the time it is simple to diagnose diabetes. A blood test called an HbA1c is done to measure the amount of glucose that has built up in your blood over a 3-month period. A high HbA1c result confirms the diagnosis. Usually this is the only test needed, but sometimes other types of glucose blood test are used. (see table below).

What is prediabetes?

The HbA1c test can also find people who have higher blood glucose levels than normal but don't yet have diabetes. This is known as prediabetes.
Some of these people will go on to develop diabetes, but others may be able to avoid getting diabetes by changing their lifestyle. If you have prediabetes, you will need to have an HbA1c test every year to check whether you have progressed to type 2 diabetes. 
Read more about prediabetes.

What blood tests are used to diagnose diabetes?

HbA1c test

HbA1c is a way of testing the average level of glucose (sugar) in your blood over the previous 8–12 weeks. It measures how much glucose has become stuck onto your red blood cells.

The HbA1c blood test can be done at any time of the day and you do not need to fast beforehand. This is why it is the usual test for screening and diagnosis of diabetes. It is also used to monitor the treatment of people who have diabetes. Read more about the HbA1c test.

Blood glucose tests

Blood glucose tests are sometimes used to diagnose diabetes. The level of glucose in your blood goes up and down depending on when and what you last ate or drank. Therefore your healthcare provider will plan the timing of your blood glucose test so they can interpret the result. There are 2 types of blood glucose blood test that are processed in a laboratory:

  • A fasting blood glucose test is done after you have not eaten or drank anything except water for 8 hours.
  • A random blood glucose test is done at any time. 

Blood glucose can also be tested on a glucose meter at home or by a healthcare worker using a drop of blood pricked from your finger. Finger-prick glucose tests are one way of monitoring diabetes but not reliable enough to make a diagnosis. 

The levels of HbA1c and blood glucose that are used to diagnose diabetes and prediabetes are given in the table below. 

Glucose tolerance test

In pregnancy, several different tests are used to diagnose diabetes. In the first part of the pregnancy, the HbA1c test can be used. Later in pregnancy, a glucose tolerance test is used. A glucose tolerance test measures your fasting blood glucose and then tests the blood glucose again 2 hours after you drink a special drink containing an exact amount of sugar.

Urine tests for glucose

People with diabetes often have sugar in their urine, but this test is not used anymore to screen for or to diagnose diabetes.

  

Normal

Pre-diabetes

Diabetes

Pregnancy diabetes

HbA1c

≤ 40 mmol/mol

41–49 mmol/mol

≥ 50 mmol/mol

 ≥ 50 mmol/mol

Fasting blood glucose

 

6.1–6.9 mmol/l

≥ 7.0 mmol/l

 ≥ 5.5 mmol/l

Random glucose

 

 

≥ 11.1 mmol/l

≥ 11.1 mmol/l

2 hours after 75g of glucose

 

7.8–11 mmol/l

≥ 11.1 mmol/l

≥ 9.0 mmol/l

What blood tests are used to monitor diabetes?

The main blood tests used to monitor diabetes are Hba1C and home blood glucose testing. Which tests you need depends on the type of diabetes you have and the medicine you are taking. 

HbA1c

HbA1c is a way of testing the average level of glucose (sugar) in your blood over the previous 8–12 weeks. It measures how much glucose has become stuck onto your red blood cells. It is a great way to see how your diabetes is responding to lifestyle choices you make and to monitor medicines. Read more about Hba1C for monitoring type 2 diabetes.

What are healthy HbA1c levels for people with diabetes?

An ideal range or target HbA1c level will vary from person to person and depends on age, type of diabetes and other health conditions or stages (such as pregnancy). Ask your doctor or nurse what your target HbA1c is. 

The following ranges provide a general guide: 

  • Less than or equal to 53 mmol/mol is a very healthy HbA1c level.
  • Between 54 mmol/mol and 63 mmol/mol is a fair HbA1c level and needs work to improve.
  • Between 64 mmol/mol and 86 mmol/mol indicates your blood glucose levels are much too high.
  • Above 86 mmol/mol indicates your blood glucose levels are extremely high and urgent action is required. 

If you are taking insulin and your HbA1c level is too low, you are at higher risk of having low blood glucose levels (‘hypos’ or hypoglycaemia). In this case, talk with your doctor/nurse about whether you need to reduce your insulin dose or frequency. 

Home blood glucose testing

If you take insulin or have diabetes in pregnancy then you usually need to check your own blood glucose level regularly at home on a drop of blood from your finger using a pricker (lancet) and a glucose meter. Your healthcare provider will tell you when you need to test, which may include:

  • before meals
  • about 2 hours after meals
  • before bed
  • sometimes overnight. 

The aim is to keep all or almost all of your blood glucose levels within a certain range. The range will be different for a blood test taken before (fasting) or after a meal. Your ranges will be personalised by your healthcare team, and this may be different depending on your type of diabetes, age and other health conditions.

For most people with diabetes, the blood glucose range is as follows:

  • Fasting blood glucose level: 4–7 mmol/l.
  • 2 hours after eating: 5-9 mmol/l for people with type 1 diabetes and under 8.5mmol/l for people with type 2 diabetes. 

Read more here about blood glucose testing in type 2 diabetes, high blood glucose and low blood glucose.

Severe hypoglycaemia is a medical emergency. Call 111 and ask for an ambulance if someone has a blood glucose level less than 4 mmol/l and any one of the following:
  • is extremely drowsy or disorientated
  • is unconscious
  • is having a convulsion (fit).

Note: If viewing American websites, watch out for different units of measurement. In the US, blood glucose is measured as mg/dL. Visit Diabetes UK to convert blood sugar/glucose from mmol/L (UK standard) to mg/dL (US standard) and vice versa.

References

  1. Type 2 Diabetes Management Guidance Ministry of Health, NZ & New Zealand Society for the Study of Diabetes, 2020
  2. Diagnosis of diabetes Auckland Regional HealthPathways, NZ, 2016

Reviewed by

Dr Alice Miller trained as a GP in the UK and has been working in New Zealand since 2013. She has undertaken extra study in diabetes, sexual and reproductive healthcare, and skin cancer medicine. Alice has a special interest in preventative health and self-care, which she is building on by studying for the Diploma of Public Health with the University of Otago in Wellington.
Credits: Health Navigator Editorial Team. Reviewed By: Dr Alice Miller, FRNZCGP, Wellington Last reviewed: 11 May 2021