Fasting Plasma Glucose Test

 

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. 

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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) 

Impaired fasting glycemia is a form of pre-diabetes.

Read more on impaired fasting glycemia.

 

HbA1c Test for Diabetes

 

Why is HbA1c important?

People with diabetes who reduced their HbA1c by less than 1% can cut their risk of dying within 5 years by 50%, according to Swedish research presented at the annual meeting of the European Association for the Study of Diabetes, Sept. 2012 (EASD).

 

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.

 

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 (see below) 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.

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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 haemoglobin, haemoglobin A1c) occurs when haemoglobin, 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 been, 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.

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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.

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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.

 

Oral Glucose Tolerance 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.

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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.

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Why is an oral glucose tolerance test done?

This can be a useful test in helping to diagnose:

• Prediabetes

• Gestational diabetes in pregnant women

• Insulin resistance

• Reactive hypoglycaemia

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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.

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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