Key Takeaways
- Some blood tests require fasting for 8 to 12 hours beforehand — always check with your GP or the testing service.
- Drinking water is almost always encouraged, even during a fast, because it makes veins easier to find.
- Wear loose-fitting clothing with sleeves that roll up easily above the elbow.
- Tell your phlebotomist about any medications, supplements, or allergies before the draw.
Why Preparation Matters
A blood test is one of the most routine medical procedures in the UK. The NHS carries out more than one billion pathology tests each year, and the vast majority involve a simple blood draw from a vein in your arm. While the procedure itself takes only a few minutes, how you prepare in the hours beforehand can influence both the accuracy of your results and the ease of the blood draw.
Proper preparation reduces the chance of haemolysed (damaged) samples, inaccurate lipid or glucose readings, and difficult venous access. It also makes the experience more comfortable for you. This guide covers everything you need to know, whether you are having blood taken at your GP surgery, an NHS hospital, or through a mobile phlebotomy service at home.
Step 1: Check Whether You Need to Fast
Fasting means not eating or drinking anything other than water for a set period — usually 8 to 12 hours — before your test. Fasting is required when the test measures substances that are directly affected by recent food intake. The most common fasting tests are:
- Fasting blood glucose — measures your blood sugar level after an overnight fast.
- Lipid profile (cholesterol) — triglycerides in particular are affected by recent meals.
- Iron studies — serum iron fluctuates significantly after eating.
- Fasting insulin — used alongside glucose to assess insulin resistance.
Many other tests, including a full blood count, thyroid function tests, and liver function tests, do not require fasting. If you are unsure, contact your GP practice or testing provider. For a detailed breakdown, see our guide on whether you can eat before a blood test.
Step 2: Stay Well Hydrated
Even if you are fasting, you should drink plenty of water. Dehydration causes veins to constrict and become harder to locate, which can lead to multiple needle attempts and increased bruising. Aim to drink at least 500ml of water in the two hours before your appointment.
Avoid alcohol for at least 24 hours before any blood test. Alcohol affects liver enzymes, blood sugar, and triglyceride levels. Even moderate consumption the evening before can skew your results.
Step 3: Review Your Medications
In most cases you should continue taking prescribed medications as normal unless your doctor has specifically told you to stop. However, it is important to tell the phlebotomist what you are taking, including over-the-counter medications, vitamins, and herbal supplements. Some substances can affect test results:
- Biotin (vitamin B7)— found in many hair and nail supplements, it can interfere with immunoassay-based tests including thyroid function and troponin.
- Iron supplements— can elevate serum iron readings. Your GP may ask you to stop supplementing 24 to 48 hours before iron studies.
- Blood thinners (e.g. warfarin, apixaban)— these do not affect test accuracy but you may bleed or bruise more easily after the draw. The phlebotomist will apply pressure for longer.
Step 4: Wear the Right Clothing
Blood is almost always drawn from a vein in the antecubital fossa — the inner crease of your elbow. Wear a short-sleeved top or a top with sleeves loose enough to roll up well above the elbow. Tight sleeves bunched around the upper arm can act as a tourniquet and affect the sample.
Step 5: Managing Needle Anxiety
Fear of needles (trypanophobia) is estimated to affect around 10% of the UK adult population. It is a recognised medical phobia and there is no reason to feel embarrassed. Here are evidence-based strategies that can help:
- Tell the phlebotomist — experienced phlebotomists are trained to help anxious patients. They may use distraction techniques, apply a numbing spray, or use a butterfly needle.
- Look away — you do not need to watch. Turn your head and focus on something else in the room.
- Applied muscle tension — if you feel faint during blood tests, repeatedly tense and release the large muscles in your legs and abdomen. This technique has strong evidence for preventing vasovagal episodes.
- Deep breathing — slow, controlled breaths (four seconds in, four seconds out) activate the parasympathetic nervous system and reduce the fight-or-flight response.
- Have your blood taken at home — a home blood test can be less stressful than a busy GP surgery or hospital phlebotomy department. You are in your own environment, with no waiting room, and the phlebotomist can take more time with you.
On the Day: What to Expect
The phlebotomist will confirm your identity, check which tests have been requested, and ask about allergies (such as latex). They will apply a tourniquet to your upper arm, clean the puncture site with an alcohol swab, and insert a needle into a vein. Blood is collected into one or more colour-coded vacutainer tubes. The whole process usually takes two to five minutes.
Afterwards, a cotton pad or plaster will be applied. Keep it on for at least 30 minutes and avoid heavy lifting with that arm for the rest of the day to minimise bruising.
After the Test
If you fasted, have a small meal or snack ready. Most people feel completely normal afterwards, but some may feel slightly light-headed. Sit down for a few minutes if this applies to you. Stay hydrated throughout the day.
Your results will be processed by a laboratory and returned to whoever ordered the test. For typical turnaround times, read our guide on how long blood test results take in the UK.
Need a blood test at home?
Lola Dispatch connects you with qualified, DBS-checked phlebotomists across the UK. Skip the waiting room and book a convenient home visit.