Preparing for a blood test: water, coffee, medicines and why in the morning

For most routine tests, 8 to 12 hours without food is usually recommended. It is normally enough to have your last meal as dinner the evening before and to come in the morning without breakfast. That way most of the fasting happens while you sleep.

Fasting matters above all for glucose (blood sugar) and the lipid profile (cholesterol and triglycerides), because a meal changes blood sugar and especially triglyceride levels for several hours after eating. For some other tests, such as a full blood count, fasting is usually not required. Since several tests are generally done from a single blood sample, it is practical to come fasting.

Keep dinner the evening before your tests light, without fatty food. Many laboratories also advise not drinking alcohol the day before. Do not fast for longer than 14 hours. Fasting for longer does not make the result more accurate, and it can change some results. If your doctor has given you different instructions, follow your doctor's instructions.

Plain water is allowed, even before fasting tests. A glass of water in the morning is actually helpful, because it usually makes it easier to take blood. All other drinks can change the results, so save them for after the blood test.

• Plain water, unflavoured, without sugar and without sweeteners: allowed.
• Flavoured water or water with lemon: not allowed.
• Coffee, even without sugar and without milk: not allowed.
• Tea, juice, milk, fizzy drinks and energy drinks: not allowed.
• Chewing gum and sweets, including sugar-free ones: not allowed.
• Smoking in the morning before the blood test: not allowed.

If a morning without coffee is hard for you, come as early as possible. You can bring your coffee and breakfast with you and have them after the blood test.

If you did eat or drink something in the morning, tell the staff before your blood is taken. Sometimes the sample can still be taken, and sometimes it is better to come back another day. Do not keep it to yourself, because a result from a test that was not done fasting can lead to the wrong conclusion.

Do not stop medicines you take every day on your own because of a blood test. For most tests, medicines are taken as usual. For some tests, your doctor may ask you to take a medicine only after the blood test, and occasionally to stop it for a while. This is always agreed with the doctor who prescribed your medicines or referred you for the tests, never decided on your own.

When you arrive, tell the laboratory staff which medicines you take, including vitamins and dietary supplements. Some supplements, such as biotin (vitamin B7), which is also found in hair, skin and nail products, can affect the results of certain tests, including thyroid hormone tests. Ask your doctor whether, and how long beforehand, you should stop taking them.

Two situations need particular care:

• Diabetes medicines (tablets or insulin): if you do not eat in the morning but take your usual dose, your blood sugar can drop too low. Ask your doctor in advance how to fit your medicine around fasting. Bring your medicine and something to eat so you can take them after the blood test. If you feel shaky, sweaty, very hungry or weak while you are waiting, tell the staff as soon as possible.
• Anticoagulants (blood thinners): if you take them, say so before the sample is taken. The puncture site then needs to be pressed for longer.

For TSH, FT4 and FT3, fasting is usually not required, but the sample is normally taken in the morning. These tests are often done on the same morning as blood glucose or blood fats, so it is practical to come fasting. Antibodies, such as anti-TPO, are usually measured from the same blood sample.

People who take thyroid hormone tablets are usually advised to take the tablet after the blood test that morning, not before. That way the result shows the level before the new dose. Since this tablet is usually taken on an empty stomach, wait as long as your doctor has told you before having breakfast and coffee. If you take other thyroid medicines or your doctor has given you different instructions, follow those instructions.

If you take biotin or hair, skin and nail products, say so before your blood is taken, because biotin can change thyroid hormone results.

When tests are repeated for monitoring, it is best to come at a similar time of day and under similar conditions. That makes it easier to compare new and previous results.

Tests related to blood sugar do not all need the same preparation:

• Fasting glucose: usually 8 to 12 hours without food, with plain water only.
• HbA1c: shows your average blood sugar over the previous two to three months and usually does not require fasting, so it can also be done later in the day.
• Insulin: usually done fasting, together with glucose.
• OGTT (glucose tolerance test, listed in the price list as the oral glucose tolerance test): done in the morning, fasting, on your doctor's recommendation. For three days before the test, eat as usual, without dieting and without cutting out carbohydrates. After the first sample, you drink a glucose solution and blood is taken again, usually after two hours, and in pregnancy often also after one hour. During this time you do not eat or smoke, and you stay at rest. If you feel unwell or vomit during the test, tell the staff as soon as possible.

For an OGTT, ask in advance whether you need to book and how much time to set aside, because the test takes longer than a standard blood test. If you already have diabetes or your blood sugar has been raised in the past, an OGTT is done only on your doctor's recommendation. If you take any diabetes medicines, agree the preparation and your morning dose with your doctor in advance.

The morning is practical for fasting tests, because most of the fasting happens overnight. In addition, the levels measured by some tests change during the day, for example iron and some hormones, such as cortisol. That is why these tests are usually done in the morning. Sex hormones are often tested on a particular day of the menstrual cycle, and that day is set by your doctor.

That morning:

• Drink a glass of plain water.
• Do not exercise or do heavy physical work before the blood test. It is usually also advised to avoid very strenuous training the day before.
• Wear clothes with sleeves that can easily be rolled up above the elbow.
• Bring your referral or list of tests, if you have one, and a list of the medicines you take.
• Before the blood test, sit down and rest for a few minutes.

If you have felt unwell during a blood test before, say so before the sample is taken. Phone ahead to check the laboratory's opening hours and whether you need to book. The phone numbers are at the bottom of this page and on the Contact page. The laboratory is in the Maja Hospital building, at Vizantijski bulevar 39 in Niš.

After the sample is taken, press on the puncture site with dry cotton wool or a gauze pad for a few minutes, and for longer if you take anticoagulants (blood thinners). It is usually best to keep your arm straight, because bending it at the elbow makes bruising more likely.

Once blood has been taken for all your fasting tests (for an OGTT, only after the last sample), take your medicines, and you can eat and have coffee, unless your doctor has told you otherwise. A thyroid hormone tablet usually needs to be taken on an empty stomach, so after taking it, wait as long as your doctor has told you before having breakfast and coffee.

• Do not carry anything heavy with that arm for a few hours.
• A small bruise is possible and usually fades within a few days.
• If you feel dizzy or weak, tell the staff and sit or lie down until it passes.
• If the puncture site keeps bleeding even after pressing, swells, turns red, becomes more painful, or you feel tingling down your arm, contact a doctor.

The time it takes to get your results depends on the type of test, so check this when your blood is taken. Prices for individual tests and for blood sampling are in the laboratory price list.