Blood tests for diabetes | Health Navigator NZ (2022)

Screening for diabetes involves tests to identify people who are at risk of having type 2 diabetes.

On this page you can find the following information:

  • Why are blood tests done to screen for diabetes?
  • Who is most at risk of developing diabetes?
  • Who should be screened for diabetes?
  • How is diabetes or prediabetes diagnosed?

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 bescreened.

Who is most at risk of developing diabetes?

There areknown 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.
  • Havinga father, brother, mother or sister who developed type 2 diabetes at an early age, e.g. < 40 years.
  • Having hadgestational diabetes.
  • Havingpolycystic ovary syndrome.
  • Taking steroid tablets long-term.
  • Havingsevere mental illness, particularlyif you are on long-term antipsychotic treatment.
  • Having heart disease or stroke.

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. Read more aboutprediabetes.

Who should be screened for diabetes?

Routinescreening

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 screening shouldstartfor menAge screeningshouldstartfor women
  • With any of the above risk factors: 18 years
  • With no known risk factors:45 years
  • Māori, Pacific or South-Asian ethnicity:30 years
  • Known cardiovascular risk factors or at high risk of developing diabetes: 35 years.
  • With any of the above risk factors: 18 years
  • With no known risk factors:55 years
  • Māori, Pacific or South-Asian ethnicity:40 years
  • Known cardiovascular risk factors or at high risk of developing diabetes: 45 years.

Why are somechildren 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 shouldhave 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 diabetesknown 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 allpregnant women should be screened for diabetes. Read more about gestational diabetes.

How is diabetes or prediabetes diagnosed?

HbA1c test

Most of the time it is simple to diagnose diabetes. A blood test called anHbA1cis 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 testcan 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 theHbA1c test.

  • If you have symptoms of diabetes (such as excessive thirst, frequent peeing, or recurrent infections),only onehigh HbA1c result (50 mmol/mol or higher) is needed to confirm the diagnosis.
  • If you do not have symptoms of diabetes, two 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 onto develop diabetes, but othersare 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 doctoror 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 bloodglucose levels are between 4.0and 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, buturine tests are nolonger usedto screen for diabetes or to diagnoseit.

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

Type of test

Normal

Pre-diabetes

Diabetes

Diabetes in pregnancy (gestational)

HbA1c

≤ 40 mmol/mol

41–49 mmol/mol

≥ 50 mmol/mol

≥ 50 mmol/mol

Fasting blood glucose

4.0and 5.4 mmol/L

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 mmol/L

7.8–11 mmol/L

≥ 11.1 mmol/L

≥ 9.0 mmol/L

References

  1. Type 2 diabetes management guidanceMinistry of Health, NZ &NZ Society for the Study of Diabetes, 2020
  2. Diagnosis of diabetes Auckland Regional HealthPathways, NZ, 2016
  3. Diabetes diagnosis Mayo Clinic, US, 2020

Reviewed by

Blood tests for diabetes | Health Navigator NZ (1)Dr Alice Millertrained 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.

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