How to prepare for a blood test: the night before and the morning

A blood check-up needs 8 to 12 hours of fasting. You can drink water; no coffee, tea or breakfast. Here is what to do the night before and in the morning, what is allowed and what is not, and how to handle medicines, your period and the urine sample.

What you can and cannot do

In short, for a fasting blood test:

These are general rules. If your doctor or the lab gives you different instructions, follow theirs.
WhatAllowed?Good to know
WaterYesDrink as usual, in the morning too. Plain water only, no sugar or flavouring.
FoodNoNothing to eat for 8 to 12 hours. Bring something for afterwards.
CoffeeNoNot even black. Have it after the draw.
Tea and herbal teaNoEven without sugar. Until the draw, water only.
Milk, juice and soft drinksNoThey contain sugar and break your fast.
Brushing your teethYesDo not swallow the toothpaste. Rinse with water afterwards.
Chewing gum and sweetsNoNot even sugar-free ones.
SmokingNoDo not smoke on the morning of the draw.
AlcoholNoNo alcohol in the 24 hours before.
Prescribed medicinesYesAs usual, unless your doctor says otherwise.
SupplementsAfter the drawFor biotin and iron, ask your doctor whether to pause them in the days before.
Hard exerciseNoNot the day before and not on the morning of the draw. A walk is fine.

How many hours to fast

Nothing to eat for 8 to 12 hours. Usually it is enough to have dinner the night before and skip breakfast.

Fasting matters because many values change in the hours after a meal: sugar and fats in your blood, but also iron, some salts and some liver enzymes. When you fast, your result can be compared with the limits on the report and with your earlier tests.

An example: if your draw is at 8 am, finish dinner between 8 and 10 pm. After that, water only.

Longer does not mean better. Skipping dinner to stretch the fast does not make the tests more accurate: a long fast can raise bilirubin, lower your glucose a little and make ketones appear in your urine.

The night before: what to eat

A normal dinner is fine. You do not need a special diet, that evening or in the days before: the test should show how you usually are.

A simple dinner without excess is best, for example:

Avoid a very fatty or very large dinner, especially if you eat late: a fatty meal raises triglycerides for a few hours. Go easy on meat, which can raise creatinine for many hours. No alcohol, and try to sleep well: even one bad night can change some values, like glucose.

The morning of the draw

Can you drink water before a blood test?

Yes. Water does not break your fast: you can drink it the night before and on the morning of the draw too.

In fact it helps. When you are well hydrated, the draw is usually easier. With too little water, some values such as creatinine can rise a little.

There is no need to overdo it: drink what you usually drink. A lot of extra water dilutes your urine and changes the urinalysis.

Plain water only: no flavoured water, sugar or sachets stirred in.

Coffee, tea and breakfast

On the morning of the draw, no coffee, not even black, and no tea.

Coffee can raise your glucose, and with milk or sugar it breaks your fast completely. Tea contains caffeine too.

The same goes for milk, fruit juice, soft drinks and herbal tea: until the draw, water only.

Bring your breakfast with you: have it as soon as you are done, with your coffee.

Brushing your teeth, chewing gum and sweets

Yes, you can brush your teeth on the morning of the draw. Do not swallow the toothpaste, and rinse with water.

Chewing gum and sweets are a different matter: skip them, even sugar-free ones. Wait until after the draw.

Smoking before the draw

Better not. Do not smoke on the morning of the draw: smoking changes some values in your blood for a while.

That goes for e-cigarettes and heated tobacco too. Wait until you are done.

If you smoke regularly, tell your doctor when they read your report: people who smoke often have slightly higher white cells.

Medicines and supplements

Keep taking your prescribed medicines as usual, unless your doctor says otherwise. Do not stop a treatment on your own for the blood test. A few cases to know about:

Take any other supplements after the draw on that morning, with breakfast.

Alcohol and exercise in the days before

No alcohol in the 24 hours before the draw, not even a glass of wine with dinner: it can raise triglycerides.

The day before and on the morning of the draw, avoid hard exercise. A tough workout, especially with weights or if you are not used to it, can raise liver enzymes and creatinine for days, even when your liver and kidneys are fine.

If you have a race or a very hard session planned, put it after the draw. A walk or your usual light activity is fine.

Blood tests during your period

You can have a blood draw during your period. For the tests in a check-up, it usually makes no difference.

Some hormones are the exception: they change with the days of your cycle, so your doctor tells you when to test them.

The urine sample is different: menstrual blood gets into the sample and the blood reading turns positive. It is best to collect it a few days after your period ends.

If your periods are often heavy, tell your doctor: in women before menopause it is the most common cause of low iron.

With Rosa you hand in your urine at the partner lab, together with the blood draw. If you can, book a few days after your period ends. If your period starts that very day, tell the lab before you hand in the sample: they will tell you what to do.

The urine sample

For a urinalysis you usually do not need to fast. If you do it together with blood tests, follow the rules for the blood draw.

In general you use your first morning urine, in a sterile container. Wash your hands, let the first stream go into the toilet and collect the middle part. Close the container tightly and hand it in soon.

The days before count here too: hard exercise can make protein or a little blood appear, and a lot of water dilutes your urine.

The partner lab tells you how and when to collect the sample: follow its instructions.

At the lab: during and after the draw

See a doctor the same day if your arm swells a lot, the bruise spreads quickly, or you have tingling or strong pain running down towards your hand that does not go away. If your arm swells fast and your hand turns cold, pale or numb, call 112 or go to the emergency department.

The tests most affected by how you prepare

If a value comes back out of range, before you worry, ask yourself how you prepared. These tests in the Rosa panel change more than the others:

Frequently asked questions

I had breakfast by mistake: can I still have the draw? Tell the lab before the draw: they will tell you whether to go ahead or whether it is better to come back another day. After a meal, glucose, insulin and triglycerides can read higher than they really are.

Which tests need fasting? Mostly glucose, insulin, triglycerides and iron, which change after a meal. Cholesterol, ApoB and lipoprotein(a) change little and can also be measured without fasting. But when many tests are done together, as in a check-up, you fast for all of them.

Why are blood tests done fasting? Because many values change in the hours after a meal. When you fast, your result can be compared with the limits on the report and with your earlier tests, done in the same conditions.

Can I take my thyroid tablet before the draw? Keep taking it as usual, unless your doctor says otherwise. Ask them whether to take it before or after the draw that day, and do not skip it without talking to them first.

Does the draw hurt? You feel a short prick that passes quickly. Keep your arm relaxed and breathe calmly. If you are scared of needles, say so beforehand: you can have it lying down.

Do I need a GP prescription with Rosa? No. Rosa prepares the test request. You print the lab’s form and sign it by hand.

Do your blood check-up with Rosa

A fasting blood draw at a partner lab, no prescription needed. 31 tests in the first draw, a re-check of your key values six months later, every value explained and a doctor who approves your report. €295 a year.

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