Diabetes – screening and diagnosis

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. 

There are known risk factors for developing diabetes, these include:

• Being overweight or obese – a BMI of ≥ 27 kg/m2 for people of Māori, Pacific or South Asian ethnicities, or ≥ 30 kg/m2 for people of other ethnicities.
• Non-European ethnicity.
• Having a father, brother, mother or sister who developed type 2 diabetes at an early age, e.g. < 40 years.
• Having had gestational diabetes.
• Having polycystic ovary syndrome.
• Taking steroid tablets long-term.
• Having severe mental illness, particularly if you are on long-term antipsychotic treatment.
• Having heart disease or stroke.
• Previous prediabetes or diabetes in remission.
• Post-transplant.

These people should have regular screening tests for diabetes and prediabetes. Prediabetes is when blood glucose levels are higher than they should be but are not high enough for a diabetes diagnosis to be made. 

Routine screening 

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. Read more about cardiovascular risk assessments. 

The checks include:

• measuring your weight
• measuring your blood pressure
• asking questions about your family history, eg, of diabetes, stroke or heart disease
• questions about smoking
• a blood test for cholesterol and fats
• a blood test for diabetes (HbA1C)

The age to start screening for diabetes depends on your risk factors including body weight, ethnicity, family history and other medical history.

Age when screening should start for men:

• If overweight or obese, with one or more additional risk factor: 18 years
• With no known risk factors: 45 years
• Māori, Pacific or Indo-Asian ethnicity: 30 years

Age when screening should start for women:

• If overweight or obese, with one or more additional risk factor: 18 years
• With no known risk factors: 55 years
• Māori, Pacific or Indo-Asian ethnicity: 40 years

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. 

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.

HbA1c test

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. 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 (stop eating for several hours) 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.

• If you have symptoms of diabetes (such as excessive thirst, frequent peeing, or recurrent infections), only one high HbA1c result (50 mmol/mol or higher) is needed to confirm the diagnosis.
• If you do not have symptoms of diabetes, 2 high HbA1c results (50 mmol/mol or higher) are needed to confirm the diagnosis.

Usually this is the only test needed, but sometimes other types of blood glucose tests 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 have diabetes. This is known as prediabetes and it is used to describe an HbA1c result in the range of 41–49 mmol/mol. Some people with prediabetes will go on to develop diabetes, but others are able to avoid getting diabetes by changing their lifestyle (diet, weight loss and exercise). If you have prediabetes, you will need to have an HbA1c test every year to check whether you have progressed to type 2 diabetes. You can ask your doctor or nurse for help in making changes to your lifestyle to limit your chance of developing diabetes. Read more about prediabetes.

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 drunk 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, either by you 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 are not reliable enough to make a diagnosis. 

For the majority of healthy individuals, normal blood glucose levels are between 4.0 and 5.4 mmol/L when fasting, and up to 7.8 mmol/L 2 hours after eating (postprandial).

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 urine tests are no longer used to screen for diabetes or to diagnose it. 

Read more about screening for diabetes.(external link)

Summary of HbA1c and blood glucose tests used to diagnose diabetes and prediabetes 

* Note that a fasting blood glucose of 4.0 to 5.4 mmol/L is considered to be normal but some people may have a result in the 5.5 to 6.0 mmol/L range. Your fasting blood glucose level isn't considered to be diagnostic of prediabetes until it reaches 6.1 mmol/L based on NZSSD guidelines(external link) and would be used in combination with an HbA1c test.