Checking for diabetes and metabolic risk with HbA1c and glucose

What the tests measure

HbA1c measures how much sugar has attached to the haemoglobin in your red blood cells. It reflects your average blood glucose over the previous 8 to 12 weeks, and it can be taken at any time of day without fasting [1].

Fasting plasma glucose measures the sugar in your blood after a night without food. Fasting insulin, measured at the same draw, shows how hard your body is working to keep that glucose level down, and the doctor reads the two together.

Why it is done and who it is for

Prediabetes affects around one in 12 people and usually causes no symptoms, so a blood test is the only way to find it [2]. Your risk rises with age, family history, carrying extra weight around the middle, poor sleep and smoking, and it is higher, and starts younger, in people of Black African, African Caribbean, Indian, Pakistani or Bangladeshi background [3].

NICE encourages a risk assessment for all adults aged 40 and over, and for people aged 25 to 39 of South Asian, Chinese, African-Caribbean, black African or other higher risk backgrounds [4]. Blood sugar also feeds into your heart risk, and NICE includes diabetes status in the checks a doctor makes before offering a statin [5].

Who it is not for

If you have new thirst, are passing urine more often, are losing weight without trying or have blurred vision, see a doctor promptly rather than waiting for a planned test [6]. If you feel severely unwell, call 999 in the UK or the local emergency number elsewhere.

HbA1c can mislead in some people. Certain anaemias and inherited haemoglobin variants change the result regardless of glucose [1]. If you are pregnant, ask your doctor which test suits you.

Benefits and limitations

Finding raised blood sugar early gives you time. In a large trial of 3,234 adults with raised blood sugar, a lifestyle programme cut new cases of type 2 diabetes by 58% and metformin by 31%, compared with placebo [7]. Finding prediabetes early is what makes that possible.

No single test is perfect, and the WHO notes that many people diagnosed by HbA1c would not be diagnosed by glucose, and the reverse, and that an HbA1c below 6.5% does not exclude diabetes found on glucose testing [1]. A raised first result without symptoms is confirmed with a second test before any diagnosis [4].

Risks

The blood draw is low risk, with a small chance of a bruise or feeling faint. A result in the prediabetes range can feel worrying, and NICE asks doctors to explain that high risk does not mean you will go on to develop diabetes [4]. A normal result can give false reassurance if your weight or family history keeps your risk high, which is why it is repeated.

How you prepare and what to expect

For HbA1c alone you can eat and drink as usual [1]. For fasting glucose and insulin, have nothing but water from the night before and come in the morning. Our clinics open early every day, so you can be tested before breakfast and before work. A clinician takes one sample from your arm in a few minutes.

• A list of your medicines and supplements, including any steroid or statin
• Previous HbA1c or glucose results, with their dates
• Any diabetes in your parents, brothers or sisters
• Your weight and waist measurement, if you know them

Results and what happens next

In the UK, an HbA1c of 42 to 47 mmol/mol is the prediabetes range and 48 mmol/mol or above is the diabetes range [2]. Below 42 mmol/mol is normal [2]. For fasting glucose, NICE treats 5.5 to 6.9 mmol/l as high risk and 7.0 mmol/l or above as possible diabetes needing a second test [4].

If you are in the prediabetes range, NICE recommends a repeat blood test at least once a year, and an intensive lifestyle-change programme [4]. If your risk is moderate, a check at least every 3 years is suggested, and every 5 years if it is low [4].

Questions to ask your doctor

These questions turn a number into a plan.

• Is my result normal, or in the prediabetes or diabetes range?
• Could anything about my blood make HbA1c less reliable for me?
• Which changes would make the most difference for me?
• When should I be tested again, and with which test?

How The Wellness can help

The Core Wellness blood test includes HbA1c alongside a lipid panel, thyroid and liver tests and vitamin D. Advanced Health adds fasting glucose and insulin, iron studies, a hormone profile and full thyroid tests, with a priority call from a GP to go through your results. Samples go to accredited laboratories, and every result is explained by a GMC registered doctor.

Our London clinics are on Baker Street in Marylebone and Eccleston Street in Belgravia, and outside London appointments are arranged with partners in 12 city pages.