Diabetes Screening: HbA1c, Fasting Glucose, and When to Get Tested

In the UK, it is estimated that hundreds of thousands of people are living with undiagnosed type 2 diabetes. A far greater number have blood sugar levels that are higher than normal, putting them at high risk of developing the condition – a state often called 'prediabetes'. Left undetected, high blood sugar can quietly damage your body, leading to serious long-term complications affecting your heart, kidneys, eyes, and nerves.

Our modern lifestyles, with changes in diet and activity levels, have increased the diabetes risk for many of us. The good news is that we have a powerful tool to fight back: early detection. Simple, effective screening tests can identify problems long before they cause irreversible harm. Finding out early gives you the crucial opportunity to make positive changes, or start treatment if needed, to protect your future health.

This guide is for anyone who is concerned about their risk, is experiencing symptoms that could be related to diabetes, or has been advised by a clinician to get tested. We will provide clear, reassuring information on the entire screening process. You will learn about the key risk factors, understand the main diagnostic blood tests like HbA1c and fasting glucose, see what the results mean, and know what steps to take next.

What is diabetes screening?

Diabetes screening is the process of using laboratory blood tests to detect diabetes or prediabetes in people who may not yet have symptoms. The goal is to identify high blood sugar levels early, when the condition is most manageable, allowing for timely interventions like lifestyle changes or medication to prevent long-term health complications.

Screening vs. monitoring

It is important to distinguish between screening and monitoring. Screening is performed to diagnose a condition in people who are at risk. In contrast, monitoring is what a person does after they have been diagnosed with diabetes to manage their condition day-to-day. Monitoring often involves using a home blood glucose meter, which uses a small drop of blood from a finger-prick to check sugar levels. These home kits are essential for managing diagnosed diabetes, but they are not used for the initial diagnosis, which requires a more accurate laboratory test.

When should you get tested for diabetes?

You should arrange to speak with a GP about a diabetes test if you are experiencing any key symptoms or if you have significant risk factors for type 2 diabetes. While there is no national screening programme for the general population in the UK, clinicians are vigilant in testing those who are at higher risk.

Symptoms of high blood sugar

The classic symptoms of high blood sugar are caused by glucose building up in your bloodstream. If you experience any of the following, you should consult a doctor:

• Going to the toilet more often than usual, especially at night
• Feeling thirsty all the time
• Feeling very tired
• Losing weight without trying to
• Blurred vision
• Cuts or sores that take a long time to heal

If you feel very unwell or the symptoms are rapidly progressing go to A+E. It's crucial to remember that type 2 diabetes can develop very slowly over many years. Some people may have no symptoms at all in the early stages, which is why understanding your personal risk is so important.

Risk factors for type 2 diabetes

Your risk of developing type 2 diabetes is higher if you have one or more of the following risk factors. Your GP will strongly consider testing if you fall into these groups:

• You are over 40 (or 25 for people of South Asian, Chinese, African Caribbean or black African origin)
• You have a close relative with diabetes (a parent, brother, or sister)
• You are overweight or have obesity (a body mass index, or BMI, over 30)
• You are of South Asian, Chinese, African Caribbean or black African origin
• You have a history of high blood pressure, heart attack, or stroke
• You have a history of gestational diabetes (diabetes during pregnancy)
• You have a severe mental health condition
• You have been diagnosed with polycystic ovary syndrome (PCOS)

Guidelines from the National Institute for Health and Care Excellence (NICE) recommend that people at high risk should be regularly assessed

Gestational diabetes screening

During pregnancy, some women develop high blood sugar, a condition known as gestational diabetes. It usually disappears after the baby is born, but it does increase your risk of developing type 2 diabetes later in life. Screening is routinely offered to pregnant women who have risk factors for gestational diabetes. The main test used is the oral glucose tolerance test (OGTT), which is typically performed between 24 and 28 weeks of pregnancy.

The NHS Health Check programme

For adults living in England aged between 40 and 74, the NHS Health Check programme offers a free check-up of your overall health. This assessment includes a diabetes risk evaluation. Depending on your risk score, your GP or practice nurse may then recommend a blood test, such as an HbA1c or fasting glucose test, to check your blood sugar levels.

Types of diabetes tests

A diabetes diagnosis is made using specific blood tests that are sent to a laboratory for analysis. Your GP will decide which test is most appropriate for you based on your symptoms, risk factors, and medical history. The three main tests used in the UK are the HbA1c test, the fasting plasma glucose test, and the oral glucose tolerance test.

HbA1c test

The HbA1c test, short for glycated haemoglobin, is the most common and preferred test for diagnosing type 2 diabetes in the UK. This test measures the amount of glucose that has attached to haemoglobin in your red blood cells. Because red blood cells live for about three months, the HbA1c test provides an excellent picture of your average blood sugar level over the past 2–3 months. A major advantage of this test is its convenience; you do not need to fast beforehand, so the blood sample can be taken at any time of day.

Fasting plasma glucose (FPG) test

The fasting plasma glucose test measures your blood sugar level at a single point in time. For this test to be accurate, you must have not eaten or drunk anything (except for plain water) for at least 8 hours before the blood sample is taken. This gives a baseline reading of the glucose in your blood when your body is in a resting state. It is a very well-established and reliable test that is often used as an alternative when an HbA1c test is not suitable for a particular patient, or sometimes to confirm a result.

Oral glucose tolerance test (OGTT)

The oral glucose tolerance test (OGTT) is a more involved test that assesses how well your body processes sugar. It requires you to have a fasting blood test first. Immediately after, you will be given a drink containing a specific amount of glucose. Your blood is then tested again two hours later. This shows how effectively your body has cleared the glucose from your bloodstream. The OGTT is the primary test used to diagnose gestational diabetes during pregnancy.

Random plasma glucose test

A random plasma glucose test measures your blood sugar at any time of day, without any preparation or fasting. This test is generally only used for diagnosis when a person is showing clear, severe symptoms of diabetes, such as those seen in a new presentation of type 1 diabetes. According to NICE guidance, a high result from a random glucose test in a symptomatic person often warrants a same-day assessment by a specialist diabetes team.

How diabetes tests compare

The right diabetes test for you depends on your individual circumstances, and your GP will choose the most appropriate one, with the main options being the HbA1c, fasting glucose, and oral glucose tolerance tests. This table summarises the main features of each diagnostic test.

Understanding your test results

Your diabetes test results are interpreted based on specific numerical thresholds for HbA1c, fasting plasma glucose, or an OGTT, which determine if your blood sugar levels are normal, in the high-risk (prediabetes) range, or in the diabetes range. A diagnosis of diabetes is often confirmed with a second, repeat test on a different day, unless you have clear and obvious symptoms of high blood sugar. A result in the 'prediabetes' or 'high risk' category is a critical warning sign that making lifestyle changes is highly recommended.

The table below shows the typical ranges used in the UK to classify blood sugar levels in adults (pregnancy excluded):

When HbA1c tests are not suitable

While the HbA1c is an excellent and convenient test for most people, there are specific situations where it can give a misleading result. The test relies on the normal lifespan of red blood cells, so any condition that affects them can interfere with the accuracy of the measurement. In these situations, your GP will choose to use a fasting glucose test or an oral glucose tolerance test (OGTT) instead to get a reliable diagnosis.

Factors affecting HbA1c results

Your doctor will consider these factors when deciding which test to use:

• Pregnancy: The significant changes in hormones and blood volume during pregnancy make the HbA1c test unreliable for diagnosing gestational diabetes. The OGTT is the recommended test.
• Suspected Type 1 Diabetes: Type 1 diabetes can develop very rapidly, sometimes over a few days or weeks. The HbA1c test, which reflects a 3-month average, is too slow to capture this rapid change and should not be used for diagnosis in children or young people, or in adults with symptoms of type 1. A plasma glucose test is needed for urgent assessment.
• Certain Blood Conditions: People with conditions that affect red blood cells, such as severe iron-deficiency anaemia or sickle cell disease, may have inaccurate HbA1c readings.
• Kidney Disease: In people with severe chronic kidney disease (stage 4 or 5), the HbA1c test can be unreliable.
• Certain Medications: Some medications, including long-term high-dose steroids or certain treatments for HIV, can interfere with the test's accuracy.

How to prepare for a diabetes test

Preparation for a diabetes blood test depends on the type of test ordered; an HbA1c test requires no preparation, while a fasting glucose or OGTT requires you to avoid eating or drinking for at least 8 hours beforehand.

Preparing for a fasting test (FPG or OGTT)

If you are scheduled for a test that requires fasting, here are the key steps to take:

1. Do not eat or drink anything except plain water for 8–10 hours before your test. This includes chewing gum, coffee, tea, and other flavoured drinks.
2. Schedule your appointment for the morning. This makes the fasting period easier to manage, as you can fast overnight.
3. Continue to take any prescribed medication as normal on the morning of your test with a sip of water, unless your GP has specifically told you not to.

What to tell your doctor before the test

To help your GP interpret your results correctly, you should always let them know about:

• Any existing medical conditions you have, especially those that can affect HbA1c results (as listed in the section above).
• All medications you are taking, including prescriptions, over-the-counter medicines, and supplements. This is because some can have an impact on blood sugar levels and test results.

Take control of your health

Understanding your diabetes risk and the screening process is the first step towards protecting your long-term health. We have seen that early screening, based on symptoms or risk factors, is vital for preventing serious complications. Simple and reliable blood tests, primarily the HbA1c and fasting glucose tests, are the cornerstones of diagnosis. Finally, remember that a result showing you are at high risk (prediabetes) is not a final diagnosis, but a critical opportunity to make positive lifestyle changes.

Knowing what to expect from testing empowers you to have a confident conversation with your doctor and to take control of your health.

If you have any symptoms or are concerned about your personal risk of diabetes, the next step is to speak with a GP. You can book a video consultation with a Doctor Care Anywhere GP to discuss your concerns and arrange for any necessary tests.