Screening, diagnosis and monitoring of diabetes

Diabetes screening is used to catch diabetes at an early stage before symptoms occur. The main test used for screening, diagnosis and monitoring of diabetes is HbA1C, but sometimes other tests are also used.

Key points about diabetes screening, diagnosis and monitoring

  1. Adults over a certain age, depending on your gender and ethnicity, should be screened every 3 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 indicates a diagnosis of 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 is also used to help monitor your blood glucose levels.

On this page we cover:

What is screening?

Many people who have diabetes don't feel unwell and don't see their doctor until their diabetes has caused a serious problem. Unfortunately, by the time these problems occur, damage to important parts of your body has happened. Effective treatment can slow down further complications but can’t usually repair this damage. This is why your doctor 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 as you get older, in certain ethnic groups and in people who are overweight, so these are the people doctors test for diabetes. 

Who should be screened for diabetes?

In New Zealand, screening for type 2 diabetes is part of the routine testing for heart and blood vessel (cardiovascular) disease, and includes checking your weight and blood pressure, and blood tests for cholesterol and fat in your blood. This is usually done at least every 3 years in adults over a certain age.

What age does screening for diabetes start?




People without symptoms or known risk factors

Age 45 years Age 55 years

Māori, Pasifika and Indo-Asian people

Age 35 years Age 45 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

Why are some children screened for diabetes?

Diabetes can also occur in children. Most children with diabetes have type 1 diabetes which is due to severe damage to the pancreas so not enough insulin is produced. 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 be screened especially if there is a family history of type 2 diabetes. 

Why do pregnant women need to be screened for diabetes?

Sometimes during pregnancy women develop a type of diabetes known as gestational diabetes. Gestational diabetes affects about 4–8% of all pregnant women. Untreated, it can lead to problems for both mother and baby. As a precaution, all pregnant women should be screened for diabetes. Read more about gestational diabetes.

How is diabetes diagnosed?

In most cases the diagnosis of diabetes is simple. A blood test called an HbA1c measures the amount of glucose that has built up in your blood over a 3-month period. A high HbA1c result confirms the diagnosis. In most cases no other test is necessary, but blood tests to measure the amount of glucose in your blood may also be used (see table below).

What is prediabetes?

The HbA1c test can also diagnose people who don't have diabetes but whose test is not completely normal. These people have prediabetes, which means that your blood glucose levels are sometimes too high to be called normal, but not often enough to have diabetes.

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 annual HbA1c test to check whether you have progressed to type 2 diabetes. Read more about prediabetes.

Blood tests used for diabetes screening and diagnosis

The HbA1c test

HbA1c reflects the average plasma glucose (sugar in your bloodstream) over the previous 8–12 weeks and measures how much glucose has become stuck onto your red blood cells.

The HbA1c test can be performed at any time of the day and does not require any special preparation such as fasting, which is why it is the preferred test for screening and diagnosis of diabetes. Read more about the HbA1c test.

Blood glucose levels

Because blood glucose levels increase after a meal and decrease between meals, it is necessary to know when you last ate for the blood glucose level in your blood to be used to diagnose diabetes. It can take a while for the blood glucose to reach its lowest point between meals. A fasting blood glucose test is a test that has been done after you have not eaten for 8 hours. A random blood glucose test is a blood test that has been performed at any time.

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 diagnose diabetes.







≤ 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

How is diabetes monitored?

Once a person has a diagnosis of diabetes, they will be asked to check their blood glucose level regularly, usually before breakfast, and about 2 hours after meals. Sometimes tests also need to be done 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 or after a meal. The range will be different for people with type 1 diabetes, type 2 diabetes and diabetes in pregnancy.

HbA1c is also used to monitor diabetes.

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

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

Note: If viewing American websites, watch out for different units. In the USA, 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.


  1. Screening and diagnosis of type 2 diabetes Ministry of Health, NZ & New Zealand Society for the Study of Diabetes, 2020
  2. Diagnosis of diabetes Auckland Regional HealthPathways, NZ, 2016
  3. Guidance on the management of type 2 diabetes 2011 New Zealand Guidelines Group, 2011

Reviewed by

Dr Jeremy Tuohy is an Obstetrician and Gynaecologist with a special interest in Maternal and Fetal Medicine. Jeremy has been a lecturer at the University of Otago, Clinical leader of Ultrasound and Maternal and Fetal Medicine at Capital and Coast DHB, and has practiced as a private obstetrician. He is currently completing his PhD in Obstetric Medicine at the Liggins Institute, University of Auckland.
Credits: Health Navigator Editorial Team. Reviewed By: Jeremy Tuohy, Researcher & Clinician, University of Auckland Last reviewed: 13 Feb 2020