For most blood tests it is best to come in the morning, fasting, which means 8–12 hours without food but after a glass of water. Regular medicines are usually taken as on any other day, and the day before it is best to avoid alcohol, strenuous exercise and heavy meals. Whether a particular test really requires fasting depends on the test itself. If in doubt, ask the referring doctor or the staff when you book.
What “fasting” means
Fasting means that usually 8–12 hours have passed since your last meal. In practice: have a light dinner by 6–7 pm and come for the blood draw in the morning without breakfast. Fasting for longer (more than 14–16 hours) is not recommended: it can distort some results and increases the risk of feeling faint.
Fasting does not mean going without fluids. You can and should drink water (still, unsweetened). When you are well hydrated, your veins are easier to see and the draw is easier. Do not drink coffee, tea, juice, milk or sweetened drinks. Chewing gum and sweets “to calm the nerves” also count as food.
Which tests require fasting and which do not
The list below is general. How to prepare for a specific test is decided by the doctor who orders it or by the laboratory, as the rules vary with the method used.
Usually fasting:
- glucose (fasting blood glucose) and insulin,
- lipid profile (total cholesterol, HDL, LDL, triglycerides),
- iron, ferritin, TIBC,
- liver function tests (ALT, AST, GGT, bilirubin), depending on the instructions,
- uric acid, urea, creatinine, often recommended fasting but not always required.
Usually no fasting needed:
- full blood count,
- TSH and thyroid hormones (though a morning sample is recommended),
- vitamin D,
- CRP,
- most tumour markers, such as PSA,
- serology (antibody tests).
If you are combining tests from both groups in one visit, come fasting. That is the simplest rule.
Time of day matters
Many parameters change over the course of the day, which is why the standard time for a blood draw is the morning, most often between 7 and 10 am. This applies in particular to:
- cortisol, which peaks in the morning and falls during the day; the laboratory gives a reference range for a specific hour,
- TSH, which fluctuates over the day,
- prolactin, which depends on the time of day, sleep, stress and exertion; it helps to sit quietly for ten to fifteen minutes before the draw,
- iron, whose level in the morning differs from the evening.
Laboratory reference ranges are set for morning samples. If you are repeating a test, come at a similar time so that the results are comparable.
Medicines and supplements
Do not stop regular medicines on your own (for blood pressure, thyroid, diabetes, anticoagulants). A few rules:
- Unless your doctor has advised otherwise, take your medicines at the usual time with water. Levothyroxine is sometimes an exception: some doctors ask patients to take it after the blood draw. Ask your treating doctor.
- For diabetes medicines and insulin, agree the dosing for a fasting test with your doctor so that your blood sugar does not drop too low.
- Biotin (vitamin B7, common in “hair and nails” supplements) can interfere with hormone assays, including TSH. A break before the test is recommended; its length depends on the dose, so ask your doctor or the laboratory.
- Before iron and ferritin tests, a break of a few days from iron supplements is often recommended. Confirm this with your doctor.
- Bring a list of all your medicines and supplements. It will help when the results are interpreted.
The day before: alcohol, exercise, diet
- Alcohol: avoid it for at least 24 hours, ideally 2–3 days. It affects liver enzymes, triglycerides and uric acid, among others.
- Strenuous exercise: skip it the day before and on the morning of the test. Training raises creatine kinase, liver enzymes and white blood cells.
- Diet: do not change it “for the test”. Eat as usual and only avoid a very fatty or heavy dinner.
- Smoking: do not smoke on the morning of the test; nicotine affects glucose, the blood count and some hormones.
- Stress and lack of sleep: they raise cortisol, prolactin and glucose. Get a good night’s sleep and arrive with time to spare.
- Infection: with a fever or a cold, a routine check-up is best postponed until you have recovered.
Women: the phase of the cycle for hormone tests
Levels of FSH, LH, oestradiol, progesterone and prolactin change over the menstrual cycle. Your doctor will usually specify a day of the cycle (for example days 2–5 for FSH and oestradiol, around day 21 for progesterone in a 28-day cycle). If you have not been told, ask before booking. Menstruation is not a reason to postpone a full blood count or biochemistry, although it can slightly affect iron and ferritin.
What to bring
- an identity document,
- a list of the tests you need or a referral (if you have one),
- your doctor’s preparation instructions (day of cycle, time),
- a list of your medicines and supplements,
- water and a snack for afterwards,
- clothing with a sleeve that rolls up easily.
What the blood draw is like
Blood is usually taken from a vein in the crook of the elbow. The person taking the sample applies a tourniquet, disinfects the skin and inserts the needle; blood flows into vacuum tubes. The whole procedure takes a few minutes and the needle itself a few seconds. If you find the sight of blood difficult or have fainted during a blood draw before, say so beforehand: the sample can be taken while you lie down.
To make the draw easier: drink water, keep warm (warm hands mean more visible veins), relax your arm and breathe calmly.
After the blood draw
Press on the dressing for 3–5 minutes without bending your elbow; this reduces the risk of bruising. People taking anticoagulants should press for longer. After a fasting test, eat and drink something. If you feel dizzy, see spots or feel weak, do not stand up: sit or lie down and tell the staff. A small bruise at the puncture site is common and fades on its own within a few days.
What it looks like at Sonar Clinic
At the Sonar Clinic sample collection point blood draws are booked for a specific time and no referral is needed. When booking, give the names of the tests and the staff will tell you whether to fast and what time to come. The sample collection fee is charged once, regardless of the number of tests: 11 zł (adults, pregnant women, older people and children aged 11 and over) or 20 zł (newborns and children up to 8 years). Prices of individual tests are listed on the sample collection point page. How and when to collect your results is explained when you book.
If you are wondering which tests are worth having regularly, read the guide Which blood tests are worth having regularly and how often?. Parents will find separate advice in Preparing a child for a blood test. Bookings by phone: +48 880 018 703. Sonar Clinic, ul. Gubińska 8A, 54-434 Wrocław.
This content is for information only and does not replace a medical consultation. If symptoms are sudden or getting worse, contact a doctor or call 112.