Blood test for diagnosing diabetes with a glucose tolerance test

Appointment required

Blood test for diagnosing diabetes with fasting glucose, glycated haemoglobin and a glucose curve after a 75 gram oral glucose load. It brings together the three laboratory criteria recognised by the guidelines and can reveal abnormalities that may not show up in a single fasting glucose. It requires an appointment and staying about two hours at the partner centre.

No prescription needed, appointment required

Results in 1-2 working days after the test

Includes: Glucose · 75 g oral glucose tolerance test · 2-hour glucose · Glycated haemoglobin. See what each one measures

What it is for

It is used for a broad assessment of whether there are laboratory criteria for prediabetes or diabetes. Fasting glucose shows the baseline value, HbA1c summarises the last few months and the glucose curve checks how the body responds to a specific glucose load.

It can be useful when a previous glucose or HbA1c result has fallen in a borderline range, when there are risk factors or when symptoms call for a more sensitive test. The diagnosis is made by a doctor using the results and the clinical context; the blood test does not make it automatically.

If the main question is insulin resistance with glucose that is still normal, the HOMA-IR blood test answers that question better and avoids staying two hours at the centre.

When it is recommended

A borderline fasting glucose or glycated haemoglobin
A family history of type 2 diabetes
Excess weight together with hypertension, high cholesterol, fatty liver or another risk factor
Polycystic ovary syndrome (PCOS) or a history of gestational diabetes
Intense thirst, passing more urine, blurred vision or unexplained weight loss
The need to confirm a borderline abnormality with an oral glucose tolerance test
It is not the protocol for diagnosing gestational diabetes during pregnancy

Clinical comment

The three tests do not provide exactly the same information. Fasting glucose is simple, HbA1c is more stable over time and the glucose curve is more sensitive for picking up some abnormalities after a glucose load. That is why we have designed this profile with all three.

As an educational reference, the guidelines consider a fasting glucose of 126 mg/dl or more, a two-hour glucose of 200 mg/dl or more, or an HbA1c of 6.5% or more to be compatible with diabetes. In the absence of clear symptoms and unequivocal hyperglycaemia, an abnormal result should be confirmed according to medical judgement.

Bear in mind that HbA1c can be less reliable in some anaemias, haemoglobinopathies, kidney failure and situations that change the lifespan of red blood cells. In those cases, direct glucose measurements become more important.

Nor does this blood test distinguish on its own between type 1 and type 2 diabetes. Antibodies and C-peptide are reserved for situations in which diabetes already exists and there are doubts about its classification. The assessment of gestational diabetes follows a different protocol, with its own weeks and cut-off points.

Dr. Antonio Mas

What is tested

Glucose: Measures fasting blood sugar, before the glucose solution is taken.
75 g oral glucose tolerance test: Consists of drinking a solution containing 75 grams of glucose. It is the basis of the oral glucose tolerance test, also known as the glucose curve.
2-hour glucose: Measures blood sugar two hours after the glucose load. It can reveal impaired glucose tolerance even when the fasting value is normal.
Glycated haemoglobin (HbA1c): Reflects average glucose over the last two or three months and does not depend only on the value on the day the sample is taken.

How to prepare

An appointment is required. The test takes about two hours and you must stay at rest at the partner centre between the two samples.

Fast for 8 to 12 hours. You may only drink water, and you must not smoke while fasting or during the test.
Eat normally for the three days before, without cutting down on carbohydrates.
Avoid exercise during the test.
If you have a fever, an acute infection or are taking corticosteroids, ask beforehand whether it should be postponed.
Continue your usual medication unless a doctor tells you otherwise.

You can add

Without another needle. It is offered when you order, so you do not have to decide now.

General blood testA general blood test with full blood count, glucose, cholesterol and kidney and liver function: the basic parameters included in any general check-up.+€59.00
General + nutritional blood testEverything in the general blood test (anaemia, kidneys, liver, cholesterol and glucose) plus iron stores, vitamin D, vitamin B12 and folic acid. It is the broadest add-on: it covers the general and the nutritional in one go.+€129.00

Related blood tests

We have other profiles in Heart and diabetes with more or fewer tests:

The most comprehensive is not always the most useful. If you are not sure, the one we recommend to start with is Cardiovascular risk blood test with ApoB and Lp(a).

Often checked together with

They are done from the same sample, so there is no extra visit.

What it is based on

The guidelines of the American Diabetes Association and the World Health Organization recognise fasting glucose, glucose two hours after a 75 gram oral glucose load and glycated haemoglobin as laboratory criteria. They also explain when a result should be confirmed and why the glucose curve can reveal abnormalities that do not show up in the other tests. Insulin and the HOMA-IR index are not part of the diagnostic criteria for diabetes. Nor are C-peptide and antibodies used to confirm that diabetes is present; they help classify it in selected cases. That is why they are not included in this protocol.

American Diabetes Association · Standards of Care in Diabetes 2026 · Diabetes Care 49(Supl 1):S27-S49
Diagnosis and Classification of Diabetes
This is the guideline that sets the three diagnostic criteria and their figures: fasting glucose of 126 mg/dl or more, glucose of 200 mg/dl or more two hours after a 75 g oral glucose load, and glycated haemoglobin of 6.5% or more. It also states that the oral glucose tolerance test is more sensitive than the other two, that without unequivocal hyperglycaemia two abnormal results are required (they can be two different tests from the same sample), and who should be screened: everyone from the age of 35, and at any age with excess weight plus a risk factor. These are the three tests in this profile, and there is no fourth.
Organización Mundial de la Salud · WHO/UCN/NCD/20.1
HEARTS D: diagnosis and management of type 2 diabetes
It sets out the same table of criteria with the WHO figures and adds two points that matter here. First, that fasting for baseline glucose and the glucose tolerance test lasts 8 to 14 hours. Second, that glycated haemoglobin can read too low or too high in haemoglobinopathies, kidney failure and some anaemias, which is the underlying reason for not relying on just one of the three tests. On confirmation it is stricter than the American guideline: in people without symptoms it prefers repeating the same test on another day.

Protocol designed by Dr. Antonio Mas, specialist in Endocrinology and Nutrition, Spanish medical licence no. 080846289, and reviewed in September 2026.

This blood test is a standardised protocol and does not replace a medical consultation. If you have symptoms, a known condition or a complex clinical situation, speak to your doctor.