Blood test: fasting or not? Preparation and results

Fasting or not: it depends on the test

“Do I need to fast before a blood test?” We are often asked this at Maison Médicale Parc Astrid in Anderlecht. The answer depends on the tests requested. Many routine tests do not require fasting, for example HbA1c, which reflects the average amount of sugar in your blood over several weeks. For others, fasting or a set time is still needed: your doctor will tell you when prescribing the test.

Cholesterol: fasting is no longer the rule

For a long time, the lipid profile (cholesterol and triglycerides) was measured after fasting. Since 2016, the European Atherosclerosis Society (EAS) and the European Federation of Clinical Chemistry and Laboratory Medicine (EFLM) have recommended routinely using non-fasting samples. After a usual meal, the values change little: at most, triglycerides rise by about 0.3 mmol/L (26 mg/dL) and LDL cholesterol falls by about 0.2 mmol/L (8 mg/dL). These changes do not matter for medical decisions.

A fasting sample may still be requested, for example if triglycerides are above 5 mmol/L (440 mg/dL) on a non-fasting sample, or if you are being followed for very high triglycerides.

Fasting blood glucose: at least 8 hours without food

To measure fasting blood glucose (the level of sugar in your blood), you must not have eaten for at least 8 hours. In practice, you are often asked not to eat after your evening meal and to come in the morning. The European recommendations on blood sampling suggest that, when a fasting test is requested, blood should be taken in the morning between 7 and 9 am, about 12 hours after the last meal.

If you have diabetes treated with insulin or certain tablets, fasting can cause hypoglycaemia (a marked drop in blood sugar). Ask your doctor how to adjust your treatment that day, and bring something to eat after the blood test.

What can you drink? What should you avoid?

• Water is allowed, even when fasting: you can drink still water as usual.
• On the morning of a fasting blood test, avoid coffee, tea, energy drinks, smoking and chewing gum.
• If possible, avoid alcohol and intense physical exercise in the 24 hours before.

Your medicines and tests at a set time

Do not change your treatment on your own. Ask your doctor whether to take your morning medicines before or after the blood test. For example, for a thyroid check while taking levothyroxine, your doctor may ask you to take your tablet only after the blood sample (see our article on hypothyroidism).

Some substances vary during the day. Cortisol, a hormone made by the adrenal glands, is highest in the morning, between 7 and 10 am, and lowest around midnight. For this kind of test, your doctor will therefore give you a precise time.

What to bring

• your doctor's prescription for the tests, if you were given it on paper;
• your electronic identity card;
• your health insurance fund (mutualité) details, for example a sticker;
• the list of your medicines, especially if you take an anticoagulant (a medicine that makes the blood thinner).

Please also tell us if you are afraid of needles or if you have ever felt faint during a blood test: we can then have you lie down.

What happens during the blood test?

At Maison Médicale Parc Astrid, blood tests are carried out by our nursing team, at the practice or at home, and the samples are taken to the laboratory. The nurse checks your identity and the test request. You sit down, ideally after a few minutes of rest. A tourniquet is sometimes placed on your arm, for less than a minute. Avoid clenching your fist or opening and closing your hand: this can distort some results, such as potassium.

Afterwards, press gently on the plaster with your arm straight, without bending it: bending the arm makes a haematoma (a bruise) more likely. Keep pressing until the bleeding stops, usually up to 2 minutes, and up to 10 minutes if you take an anticoagulant. Rest for a few minutes before leaving and, if you were fasting, eat something.

Fainting and bruising: good to know, no need to worry

A vasovagal reaction is a reflex response of the body, often triggered by stress or fear of the needle: sweating, pallor, feeling light-headed, sometimes a brief loss of consciousness. Tell the nurse straight away if you feel unwell: they will have you lie down, and it usually passes quickly. A small bruise at the puncture site is common and harmless. Contact us if your arm swells noticeably, becomes very painful, or if you feel tingling or loss of strength. In a life-threatening emergency, call 112; outside our opening hours, call the GP on duty on 1733.

When and how do you get your results?

The laboratory sends the results securely and electronically to the doctor who prescribed the tests. Most routine tests are available within a few days; some more specialised tests take longer. Your doctor will then let you know the next steps.

In the Brussels Region, you can also view some results online yourself, if you have given your consent to the sharing of your health data: on the Brussels Health Network (Réseau Santé Bruxellois) portal or via Masanté/MyHealth (website and app), using a digital key such as itsme or your electronic identity card. Each care provider decides what to publish and from when; some sensitive documents only become visible after an appointment with your doctor.

Why your doctor explains your results

Next to each result are reference values. They are set so that 95% of healthy people fall within them: for any given test, about 1 healthy person in 20 therefore has a result outside these values. The more tests are done, the more likely it is that one of them will be flagged as “out of range”. An out-of-range result does not necessarily mean a disease, and a normal result does not rule everything out.

Your doctor interprets your results taking into account your age, your symptoms, your treatments, your previous results and the conditions of the blood test (fasting or not, time of day). For cholesterol, for example, the desirable value depends on your overall cardiovascular risk, not just on the printed reference values. If needed, your doctor will suggest a repeat test or a further examination, without adding unnecessary tests.

Blood tests at home

Are you unable to travel because of your age, an illness or a disability? Our nursing care team can take the blood sample at your home, with the same preparation rules. Talk to your general practitioner. For the times of blood tests at the practice, please ask our reception. Not registered yet? See our new patient page.

Sources

• EAS/EFLM – Fasting is not routinely required for determination of a lipid profile (2016)
• EFLM-COLABIOCLI – Joint recommendation for venous blood sampling (2018; French translation 2019)
• WHO – WHO guidelines on drawing blood: best practices in phlebotomy (2010)
• Louvain Médical – Dosage du cortisol plasmatique : utilité et pièges diagnostiques (2017)
• British Journal of Biomedical Science – The role and limitations of the reference interval (2024)
• Brussels Health Network – Which documents are shared? (accessed 2026)
• INAMI/RIZIV – Masanté: view and manage your health data online (accessed 2026)