How to prepare for a blood test: a practical checklist

The first step is to check what has been requested. The NHS explains that blood tests are done for many reasons, such as checking general health, finding out why you have symptoms, or monitoring a condition, and that the healthcare professional who arranges the test should tell you how to book and whether to prepare. If you were not told anything, it is reasonable to ask, because the answer differs from test to test.

Lab Tests Online UK gives a useful baseline. For a full blood count it says you do not need to do anything special, and for a cholesterol test it says no special preparation is needed and that you can eat and drink normally, although if a full lipid profile is requested alongside you may be asked to fast for 8 to 12 hours. For bilirubin, a liver function marker, it also says no special preparation is needed.

Ashford and St Peter's Hospitals explains why it matters: the instructions you are given may affect the result of the test, and not following them can lead to delay or the need for blood to be taken again. If your request form lists a fasting time, a time-of-day restriction or a special handling instruction, treat it as part of the test.

The NHS says that for some tests you may need to avoid eating or drinking anything other than water, known as fasting, for a short time beforehand. East Kent Hospitals describes the practical pattern: you can eat dinner the night before, you do not eat breakfast on the morning of the test, and you drink only water on the morning of the test.

The length of the fast depends on the test and, to a degree, on the laboratory. East Kent lists examples such as 8 hours for a fasting glucose and 12 to 14 hours for lipids, whereas University Hospitals Plymouth describes a fasting test generally as 10 to 12 hours and East Sussex Healthcare lists 14 hours for fasting glucose in people without diabetes. Because the numbers differ, use the figure on your own form rather than one remembered from elsewhere.

During the fast the sources agree on the main restrictions: plain water only, no tea or coffee, including black coffee or sugar-free drinks, no fizzy drinks, no chewing gum or mints, and no smoking or vaping. If you have diabetes, East Kent says to speak to your doctor before you begin fasting for any test.

The NHS gives an example of a medicine that may need to be stopped for a short time before some tests: anticoagulant medicines used to help prevent blood clots. It does not say that everyone should stop anything, and neither do the hospital leaflets. East Kent says do not change your medication unless your doctor tells you otherwise.

The practical advice is to take your usual medicines unless the instructions say otherwise, and to say what you take when the test is arranged and again to the person taking the blood. Several leaflets ask you to mention blood-thinning medicines such as warfarin or aspirin, because they make bruising more likely and may mean the person presses for longer afterwards.

Also mention any previous problem: difficulty getting blood in the past, fainting, an arm that should not be used, or a preference for one arm. Northern Care Alliance asks you to say if you are afraid of needles or of the sight of blood. The person taking the blood has heard all of these before and can adjust how they do it.

• Tell them about anticoagulant or antiplatelet medicines.

• Mention a history of fainting or feeling unwell during blood tests.

• Say if you have had difficulty finding veins before.

• Let them know if you are allergic to plasters or tape.

Wear clothing that lets the phlebotomist reach both arms if needed. East Sussex Healthcare advises loose clothing, and Plymouth suggests removing coats or jumpers before entering to make access easier and faster. Bring your request form, and if you do not have one ask the service that arranged the test how it should be provided.

Hydration is the one thing that several leaflets recommend actively. Plymouth says that, unless a fasting test has been requested, drinking plenty of fluids before queuing helps, with water ideal because tea and coffee dehydrate you. East Sussex says that increasing plain water in the hours before can improve the process especially for people who know they can be difficult to bleed. The Wensum Valley practice advises drinking plenty of fluids the night before and the morning of the appointment. Water is permitted during a fast.

If you are anxious, say so. Ashford and St Peter's suggests bringing headphones, bringing a friend or relative, and, if you are concerned about pain, topical numbing cream that can be bought from a pharmacy and applied to the inside of both elbows an hour before. The NHS adds that you can tell the person doing the test if you are worried about needles and bring someone with you for support.

A blood test usually takes about 10 minutes, according to the NHS, and the sample is usually taken from the inside of the elbow, sometimes from the wrist or finger. Afterwards you will usually be given cotton wool or a plaster. Pressing on the site until bleeding stops reduces bruising; the leaflets suggest not carrying heavy bags with that arm for a short while.

If you felt faint, Ashford and St Peter's advises resting and having something to eat and drink before you leave when you feel well enough. If you have been fasting, it is sensible to bring something to eat for afterwards. Results are sent to the clinician who requested the test, and the NHS says they may be available after a few days and usually within a few weeks.