A fasting plasma glucose test, also known as a fasting glucose test (FGT), is a test that can be used to help diagnose diabetes or

pre-diabetes

The test is a simple blood test taken after several hours of fasting.

How a fasting glucose test is performed

A fasting glucose test will be performed in the morning as this provides the body with adequate time to fast.

The NHS advises people who are having a fasting glucose test not to eat or drink anything except water for 8 to 10 hours before the test is performed.

The test requires a blood sample to be taken from the patients arm.

Fasting glucose test results

The World Health Organisations defines the following fasting glucose test results:

• 
  Normal:
  
  Below 5.5 mmol/l (100 mg/dl)
• 
  Impaired fasting glucose:
  
  Between 5.5 and 6.9 mmol/l (between 100 mg/dl and 125 mg/dl)
• 
  Diabetic:
  
  7.0 mmol/l and above (126 mg/dl and above)
  
  
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Impaired fasting glycemia is a form of pre-diabetes.

Read more on

impaired fasting glycemia

The HbA1c test, also known as the haemoglobin A1c or glycated haemoglobin test, is an important blood test that gives a good indication of

how well your diabetes is being controlled

Together with the fasting plasma glucose test, the HbA1c test is one of the main ways in which

type 2 diabetes
is diagnosed.

HbA1c tests are not the primary diagnostic test for

type 1 diabetes
but may sometimes be used together with other tests.

For HbA1c guidelines for monitoring diabetes control, see our

HbA1c targets page

HbA1c testing in diagnosing diabetes

The World Health Organisation (WHO) suggests the following diagnostic guidelines for diabetes:

• 
  HbA1c below 42 mmol/mol (6.0%):
  
  Non-diabetic
• 
  HbA1c between 42 and 47 mmol/mol (6.0–6.4%):
  
  Impaired glucose regulation (IGR) or
  Prediabetes
• 
  HbA1c of 48 mmol/mol (6.5%) or over:
  
  
  Type 2 diabetes

If your HbA1c test returns a reading of

6.0–6.4%
, that indicates prediabetes. Your doctor should work with you to suggest appropriate lifestyle changes that could reduce your risk of developing type 2 diabetes.

HbA1c is

not

used to diagnose

gestational diabetes
in the UK. Instead, an

oral glucose tolerance test
is used.

A random blood glucose test will usually be used to diagnose type 1 diabetes. However, in some cases, an HbA1c test may be used to support a diagnosis of type 1 diabetes.

How is HbA1c tested?

To measure a person’s HbA1c level, a blood sample is taken from the patient’s arm, and used to produce a reading. In some cases, such as with HbA1c testing for children, a single droplet of blood may only be required to find out how much haemoglobin A1c is present.

How does the HBA1c test work?

HbA1c (glycated haemoglobi, haemoglobin A1c) occurs when haemoglobi, the oxygen-carrying protein in

red blood cells, becomes bonded with glucose in the bloodstream. The bonding with glucose is called glycation.

The higher a person’s blood glucose levels have bee, the higher the number of red blood cells that will have become glycated, and therefore the higher HbA1c level they will have.

Note that red blood cells exist in the body for around 3 months, therefore an HbA1c levels generally reflects a person’s blood glucose levels over the previous 8-12 weeks.

Limitations of HbA1c tests

Whilst HbA1c tests are usually reliable, there are some limitations to the accuracy of the test. For example, people with forms of anaemia may not have sufficient haemoglobin for the test to be accurate and may need to have a fructosamine test instead.

Being pregnant
or having an uncommon form of haemoglobin (known as a haemoglobin variant) can also return an inaccurate HbA1c, while readings can also be affected by short term issues such as illness as they can cause a temporary rise in blood glucose.

Because of the way the HbA1c test measures blood sugar, if you have higher blood sugar levels in the weeks leading up to your HbA1c test, this will have a greater impact on your test result than your glucose levels 2 to 3 months before the test.

The Glucose Tolerance Test (GTT), also referred to as the Oral Glucose Tolerance Test (OGTT), is a method which can help to diagnose instances of diabetes mellitus or

insulin resistance

The test is a more substantial indicator of diabetes than finger prick testing.

What is an OGT test?

The test is used to determine whether the body has difficulty metabolising intake of

sugar/carbohydrate

The patient is asked to take a glucose drink and their

blood glucose level
is measured before and at intervals after the sugary drink is taken.

Why is an oral glucose tolerance test done?

This can be a useful test in helping to diagnose:

• 
  Pre-diabetes
• 
  Gestational diabetes in pregnant women
• 
  Insulin resistance
• Reactive
  
  hypoglycemia

How is the test performed?

Before the test you will be asked not to eat, or drink certain fluids, for up to 8 to 12 hours before the test.

You may be asked to not take certain

medications
in the lead up to the test, but only if these would affect the test results.

For the test itself, you will first have blood taken to

measure your blood glucose level
before the test.
The next stage is to take a very sweet tasting, glucose drink.

Further blood samples will then be taken either at regular intervals of say 30 or 60 minutes or a single test after 2 hours. The test could take up to 3 hours.

Between blood tests you will need to wait so it’s best to have some reading material, or something else to keep you occupied, with you.

What should the OGTT results be?

People without diabetes

• 
  Fasting value (before test):
  
  under 6 mmol/L
• 
  At 2 hours:
  
  under 7.8 mmol/L

People with impaired glucose tolerance (IGT)

• 
  Fasting value (before test):
  
  6.0 to 7.0 mmol/L
• 
  At 2 hours:
  
  7.9 to 11.0 mmol/L

Diabetic levels

• 
  Fasting value (before test):
  
  over 7.0 mmol/L
• 
  At 2 hours:
  
  over 11.0 mmol/L

What do these OGT test results mean?

If you are within the impaired glucose tolerance range, you will likely be advised to make lifestyle changes. In some cases, blood glucose lowering medication may be advised.

If you fall within the

diabetic range, it is quite likely that blood glucose medication will be prescribed to help your body keep your blood glucose levels down.

A random plasma glucose test is a good way for doctors get a at short notice.

The test is not so good at diagnosing diabetes in people with mildly elevated blood sugar levels but it is good for those that have a

high blood sugar level
and may need treatment more urgently.

A random plasma glucose test therefore a useful test for diagnosing type 1 diabetes in which a delay in diagnosis could lead to a high risk of a dangerous condition known as

ketoacidosis
developing.

What does a random plasma glucose test involve?

A blood sample will be taken and analysed for the concentration of

glucose in the blood

There is no need to fast for a random plasma glucose test.

How are the results interpreted?

It is important to note that a negative result does not necessarily rule out diabetes. If your doctor suspects diabetes may be present, despite a result under 11.1 mmol/L, he or she advise

taking further diagnosis tests
to confirm or rule out diabetes.

What happens if diabetes is diagnosed?

In children and young people, a diagnosis of diabetes should involve same day referral to a specialist diabetes team. Immediate care may also be required if blood glucose is significantly above 11.1 mmol/L.


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In older people, diabetes tends to develop less quickly and therefore there is less chance of immediate danger. You should be seen to discuss your treatment promptly but same day treatment will not usually be necessary.

Diabetes tests are the best way to determine whether you are at risk of developing

type 2 diabetes
or

type 1 diabetes

There are several types of tests for diabetes including diabetes screening tests, C-peptide tests, oral glucose tolerance test, GAD antibody test and type 2 risk indicator tests.

Once you know and understand the

symptoms of diabetes, you may find that you have a higher than average diabetes risk.

Early diabetes diagnosis can help to identify and prevent diabetes complications.

Type 2 Diabetes Risk Indicator

The following test is not intended to replace diabetes information and a test from a

healthcare professional

If you believe you may have diabetes, see a doctor or visit your local healthcare centre as soon as possible. It simply gives an indication of whether or not you have a risk of developing diabetes.

To get an accurate score, answer all questions in the diabetes test and click the calculate button to get your results.

If you have any questions regarding your result, ask a question in the

Diabetes Forum

The following questions are questions that are regularly asked in regards to diabetes testing.

Are diabetes screening tests foolproof?

No, diabetes screening tests are not 100 per cent accurate.

What stops the diabetes screening test from working?

Some factors interfere with diabetes screening tests. These include taking Vitamin C (leave for 24 hours before testing.) Woman should wait until their period has finished before taking the test.

If the test reads negative, am I healthy?

If a diabetic screening test is negative, there is no assurance that you are healthy unfortunately. Furthermore, it is not certain that you do not have diabetes.

What changes should I make if my diabetes screening test reads positive?

If your diabetes screening test reads positive, you should make an appointment at once to see a doctor. Before making any changes, seek professional advice.

If I have tested negative in a diabetes screening test, should I test again?

Those people with one or more risk factors should be tested for diabetes every three years, according to experts. For those with no diabetes symptoms, once every five years is the suggested period.

A C-peptide test is a blood test which is carried out to find out how much insulin your body is producing. This may be useful for determining whether you have

type 1
or

type 2 diabetes
or whether you have insulin resistance.

What is C-peptide?

C-peptide is released at the same time as insulin.

For each molecule of

insulin
produced there is a molecule of c-peptide. C-peptide does not itself influence blood sugar.

C-peptide is a useful marker of insulin production because c-peptide tends to remain in the blood longer than insulin.

Why is a c-peptide test done?

A c-peptide test can be performed for any of the following reasons:

• To distinguish whether you may have type 1 or type 2 diabetes
• To investigate whether you have
  
  insulin resistance
• To ascertain causes of
  
  hypoglycemia
  (low blood glucose levels)
• To monitor insulin production after the removal of a tumour of the pancreas (insulinoma)

How is the test performed?

To measure level of c-peptide a

fasting blood test
is taken.

You will be asked not to eat or drink (certain fluids) for 8 to 12 hours before the test.

If you take blood glucose lowering medication you will likely be asked to stop taking these in the run up to the test.

For the blood test itself, a sample of blood will be taken from your arm and it shouldn’t take much more than a minute.

Levels of c-peptide will be measured as well as the

blood glucose level

Stimulated c-peptide test

If the test is to diagnose either type 1 or type 2 diabetes, an injection of glucagon may be given into the arm between two blood samples to attempt to stimulate

insulin production

C-peptide test results

C-peptide test results are classed within 3 ranges/values.

Normal range

• 
  The normal range for a c-peptide test is:
  
  0.51 to 2.72 nanograms per millilitre (ng/mL)
• 
  This may also be expressed as:
  
  0.17-0.90 nanomoles per litre (nmol/L)

The following information serves only as a guide. Your

doctor
should be able to advise you as to what the test results may indicate.

Low test result values

Low levels of c-peptide and

high blood glucose levels
could be an indicator of type 1 diabetes.

Low levels of both c-peptide and blood glucose could indicate
liver disease, a severe infection or Addison’s disease.

High test result values

High levels of c-peptide with a low level of blood glucose could be an indication of insulin resistance, either type 2 diabetes or Cushing’s syndrome.

High levels of c-peptide but low

blood glucose
levels may be a result of insulinoma (a tumour of the pancreas) unless glucose lowering medication has influenced the result.

Diabetes screening tests are a good preventative method for catching the development of diabetes at an early stage.

Diabetes screening tests are becoming more commonly used, in the UK, as

diabetes prevalence
continues to grow.

What should a diabetes screening test be used for?

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A results from a diabetes screening test can be used to seek further medical advice from a doctor or medical professional.

The screening test is not intended as a

diagnosis, and diabetes screening test are not always accurate. If, at any stage of the test, you are nervous or unsure, contact your doctor for further advice.

• Find out more about
  
  prediabetes tests

What tests currently exist for diagnosing diabetes?

The following tests are currently used to diagnose or distinguish between certain types of diabetes:

Diabetes Tests

C-peptide Test

Plasma Glucose Test

GAD Antibodies

HbA1c Test

Oral Glucose Tolerance

Diabetes Test

Should I be screened for diabetes?

If you are aged between 40 and 74, you are eligible to receive an NHS Health Check, which checks for the presence or risk of type 2 diabetes and other common long term health conditions.

A screening test is advisable if you have any of the

symptoms of diabetes, such as frequently needing to urinate, increased thirst or persistent lethargy.

Diabetes screening tests
are also advisable for people with a number of risk factors for type 2 diabetes such as being overweight or obese, having a close family member with type 2 diabetes or being of African-Caribbea, Middle Eastern or

South Asian origin

Screening for

gestational diabetes
should either occur between weeks 24 and 28 weeks of pregnancy or, if you have had gestational diabetes in a previous pregnancy, between 16 and 18 weeks and, if negative, followed by a second test at 28 weeks.

NHS Health check

If you are between 40 and 74 years old, the NHS Health Check is a great way to assess your risk for a number of long term health conditions such as:

• 
  Type 2 diabetes
• 
  Heart disease
• 
  Stroke
• 
  Kidney disease
• Some forms of dementia, such as
  
  Alzheimer’s disease

The NHS Health Check involves completing a health questionnaire and a number of tests will be performed, including a blood pressure check, having your height and weight measured and having a blood test.

What should I do if I have a positive result for a diabetes screening test?

A positive result will usually indicate a diagnosis of diabetes, however, if there is doubt, you may be given a second test to confirm the result.

If you fall within the

prediabetes
range of a diabetes test, you should be monitored with a further diabetes screening test each year.

What should I do if I have a negative result for a diabetes screening test?

A negative result for a diabetes screening test is good news, however, it’s important to be aware that diabetes can develop later on in life so it is important to be aware of the symptoms of diabetes. If you spot the symptoms of diabetes, contact your GP.

It is also advisable to reduce your risk of developing type 2 diabetes by maintaining a healthy diet and taking regular physical activity.