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What not to do before a blood test?

Overview — Blood tests

Blood tests have a wide range of uses and therefore are one of the very commonly ordered medical tests.

A blood test is useful for:

  • checking your general health
  • checking if you have an infection
  • seeing how well certain organs are working — such as the liver and kidneys
  • screening for certain genetic conditions

A blood sample will be needed for your blood tests.

This will usually be taken by a :

  • doctor
  • nurse
  • phlebotomist (a specialist in taking blood samples)

Preparing for a blood test

The healthcare professional who arranges for your blood sample to be taken will tell you if you need to do anything before your test.

Depending on the type of blood test, you may need to:

  • avoid eating or drinking anything (fasting), except water for up to 12 hours
  • stop taking a certain medication

Follow the instructions you’re given

If you do not, it may affect the result of your blood test. It could also mean that your blood test is delayed or needs to be repeated.

During a blood test

A blood test usually involves you getting a blood sample taken from a blood vessel in your arm.

The usual place your doctor or nurse will take a sample from is the blood vessel inside of the elbow or wrist. This is because the veins here are close to the surface.

A tourniquet (tight band) is usually put around your upper arm. This squeezes the arm, temporarily slowing down the flow of blood. It causes the vein to swell. This makes it easier for your health professional to take a sample.

A needle attached to a syringe or special container is inserted into the vein. The syringe draws out a sample of your blood. You may feel a slight pricking or scratching sensation as the needle goes in, but it shouldn’t be painful.

When they take the sample, they will remove the needle. They will apply some pressure to your skin for a few minutes using cotton-wool. They may put a plaster on the small wound to keep it clean.

If you feel nervous about needles and blood, tell the person who is taking the sample. They can make you feel more comfortable. You can lie down for the test.

Blood tests for children

Blood samples from children are often taken from the back of the hand. Their skin may be numbed with a special spray or cream before the sample is taken.

After the test

Only a small amount of blood is taken during the blood test. You should not feel any significant after-effects.

But some people feel dizzy and faint during and after the test. If this has happened to you in the past, tell the person carrying out the test. It’s important they know so that they can look after you.

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After the test, you may have a small bruise where the needle went in. Bruises can be painful, but are usually harmless and fade over the next few days.

Blood test results

After you give a blood sample, it will be put into a bottle and labelled with your name and details.

It will then be sent to a laboratory for testing. A specialist will examine the sample under a microscope. They may also test it with chemicals. It depends on what they are looking for.

They will send the results to the hospital or to your GP. Some test results will be ready the same day or a few days later. Others may not be available for a few weeks. Your doctor or nurse will tell you when your results will be ready and how you’ll get them.

Sometimes, receiving results can be stressful and upsetting. If you’re worried about the outcome of a test, take a trusted friend or relative with you. For some tests, such as HIV or genetic testing, you will be offered specialist counselling to help you deal with your results.

Content supplied by the NHS and adapted for Ireland by the HSE

Page last reviewed: 22 December 2020
Next review due: 22 December 2023

This project has received funding from the Government of Ireland’s Sláintecare Integration Fund 2019 under Grant Agreement Number 123.

How to prepare for a blood test

A blood test is one of the most common medical tests and is a good way to get a picture of your overall health, which helps medical professionals assess health and disease. Most people will likely need a blood test at some point in their lives so it’s important to know how to prepare.

Last updated by

Last updated 19 Mar 2021
6 mins read

Let’s face it, most of us don’t enjoy going for a blood test, no matter how simple it is. Dr Deborah Lee, of Dr Fox Online Pharmacy, explains how to prepare.

«When you need to have a blood test, it’s important to realise that there are many different types of blood tests,» she says.

«You will be given specific instructions about your blood test, such as whether you can eat and drink beforehand, or what time of day it needs to be taken. It’s in your best interests to follow these instructions carefully because this will affect your results.»

To snack or not to snack?

If you’ve been told you need a blood test it’s likely you’ve been told whether it’s a fasting or non-fasting test. If it’s a fasting blood test, you’ll need to avoid eating and drinking (except water) for a period of time before the test is performed. Your doctor will give you this information ahead of the test.

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For example, if you’re being tested for blood glucose, lipid profile or iron levels you may need to avoid eating and drinking beforehand, Dr Lee says. Fasting is usually about 8-10 hours, so blood tests are often done in the morning before you’ve had your breakfast.

«The most common blood tests can be taken at any time. A good example is a full blood count, which checks the number of red cells, white cells, and platelets,» Dr Lee explains.

«However, some blood tests — for example, a blood glucose level — are affected by whether you have recently been eating or drinking. In this case, you are usually asked to fast — meaning you must not eat or drink before having your blood taken.»

Why do you have to fast for some blood tests but not others?

To simplify a complicated physiological answer, eating and drinking affects the composition of your blood which is why some blood tests require you to fast beforehand.

Take, for example, your blood glucose level. When we eat, our body releases a hormone called insulin which lowers our blood glucose (sugar) levels and allows our cells to absorb the glucose for energy.

But that has an impact on the body’s usual blood glucose level, so it makes it hard for your doctor to get an accurate reading on whether your glucose levels are in a healthy range. That’s why they might ask you to fast beforehand — to better understand how your body is dealing with sugars. This is often required to test for diabetes.

It’s a similar story for other tests that require fasting, as the nutrients you absorb through food and drink can impact the results of your blood test.

Medical mystery

If you’re taking regular medication you should speak with your doctor about whether it will affect your test results.

«In general, you should continue taking your medication as usual if you are having a blood test. If your medication needs to be stopped or taken at a different time, your doctor will inform you,» Dr Lee explains.

But some medications can impact your test results. Dr Lee lists these examples:

  • Antibiotics — penicillin can cause falsely elevated blood glucose results; cotrimoxazole and erythromycin can affect blood clotting times; and levofloxacin and ofloxacin can affect urine screening for opiates.
  • Antidepressants and antipsychotics can cause false positive blood pregnancy test results.
  • Beta-blockers can reduce HDL (good) cholesterol and raise triglycerides, a fatty acid tested for in cholesterol tests.
  • Steroids — oral prednisolone can inflate HDL (good) and LHL (bad) cholesterol, and triglycerides.
  • Herbal remedies, vitamins, and supplements can affect liver function and alter blood test results.
  • If you’re taking lithium and need a blood test to check your lithium levels, the test should be taken 12 hours after your last dose.
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Speak with your doctor before you stop taking these medications or if you are concerned they could affect your blood test results.

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Needle nerves

It’s not unusual to feel nervous before a blood test. In fact, a fear of needles affects 3-10% of the population, Dr Lee says.

But if you’re not needle phobic and you’re just feeling a bit nervous, there are a few things you can do to feel calmer before your test, Dr Lee explains.

  • Take someone with you to hold your hand (although this is unlikely to be allowed at the moment due to the pandemic).
  • Tell the doctor, nurse or phlebotomist what you’re worried about. They’ve seen it all before and can reassure you.
  • Drink water before your test to keep yourself hydrated.
  • Wear a warm jumper. Keeping your arms warm helps make your veins more visible.
  • Distract yourself. You could, for example, listen to music while the test is being taken.

«When you start to feel anxious, this switches on your ‘fight, fright and flight’ mechanism — your body thinks it is in danger and is prepared for action,» Dr Lee says.

«You start to feel hot and sweaty with a racing pulse. You may have a fall in blood pressure. Your pupils will be dilated. Often people feel light-headed and dizzy — they may feel faint and collapse. This is because your sympathetic nervous system (SNS) has swung into action.»

The good news is, there are ways to manage it. A breathing technique called diaphragmatic breathing can help calm your nerves, according to Dr Lee.

«Diaphragmatic breathing means using your diaphragm to fill your lungs. If you consciously try to suck in your abdomen, you will be raising your diaphragm,» Dr Lee explains. «Practise this at home, the night before, and the morning of your blood test. You can even be doing it in the waiting room while waiting for your test.»

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Feeling faint

It’s not uncommon to feel dizzy, or even to faint, when going for a blood test. And the problem often gets worse if you’re scared of needles. If you do feel dizzy or as if you might faint, it’s important you tell your doctor. They can then make you feel more comfortable, or set you up somewhere safe in case you do faint during the test.

Dr Lee suggests focusing on something other than the needle. «Focus on an object in the room and think hard about that one thing. Concentrate on it — focus your mind on this practical thing in front of you and ignore the blood test as much as you can,» she adds.

If you’re someone who is prone to feeling woozy or dizzy once your blood has been taken, it might be worthwhile packing something to eat following the test, provided your doctor says it’s OK.

There’s a reason you get a cookie after donating blood — it helps get your blood sugar levels back up and will stop you feely woozy. Packing a bottle of water or juice can also be helpful to replenish the fluid you lost due to the blood test.

And remember, your doctor will be able to answer any questions you have when you’re booked for a blood test, so don’t be afraid to ask.

Blood Test

A doctor selects a test tube of blood to examine.

A doctor selects a test tube of blood to examine.

A blood test is a lab analysis of things that may be found in your blood. You may have blood tests to keep track of how well you are managing a condition such as diabetes or high cholesterol. You may also have them for routine checkups or when you are ill.

Blood tests are very common. They are ordered by healthcare providers to:

  • Find out how well organs such as your kidneys, liver, heart, or thyroid are working
  • Help diagnose diseases such as cancer, diabetes, coronary heart disease, and HIV/AIDS
  • Find out if your medicine is working to make you better
  • Diagnose bleeding or clotting disorders
  • Find out if your immune system is having problems fighting infections
  • Diagnose anemia, such as iron-deficiency anemia, pernicious anemia, aplastic anemia, or hemolytic anemia
  • Find variations in hemoglobin such as hemoglobin S, C, or E, which are common in people of African, Mediterranean, or Southeast Asian background
  • Monitor chronic health conditions and diseases
  • Find health problems in their early stages

You have the right to know why a blood test has been ordered. Ask your healthcare provider if you are not sure why he or she wants you to have the test.

Types of blood tests

These are common blood tests:

  • Complete blood count, also called a CBC
  • Blood chemistry tests
  • Blood enzyme tests
  • Blood tests for heart disease risk
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Blood tests can give your healthcare provider a lot of information. He or she can see if certain elements in your blood are in a normal range. But in many cases, blood tests are only part of the information your healthcare provider needs to make a diagnosis of a health condition. You might need to have some other types of tests as well.

Preparing for a blood test

For most kinds of blood tests, you don’t need to prepare. These tests are to see what your blood is like under normal conditions.

For some blood tests, you will have to not eat (fast) for a certain amount of time before the blood test. This usually means no eating or drinking anything after midnight before the test. These tests are often scheduled for early in the morning.

Your healthcare provider will let you know if you need to fast before a blood test.

The procedure

In order to test your blood, a technician called a phlebotomist will use a needle to take a sample of blood. Tell the technician if the sight of needles makes you nervous. He or she can help you feel more at ease. You can also look away during the procedure, and bring a family member or friend to help distract you.

In most cases, the sample is taken from a vein in your arm. You will be seated or lying down. You may be asked to make a fist. The technician will tie a rubber band around your arm. Once he or she sees a vein, the technician will clean the area and then insert the needle. You might feel a small prick. Once the technician has drawn enough blood, he or she will take the needle out and put an adhesive bandage over the site. You may be asked to press firmly on the site to stop any bleeding.

After the procedure

Your blood sample will be sent to a lab. Trained technicians then look for the information the healthcare provider has ordered. This may take a day or up to a week or more. Check back with your healthcare provider’s office to find out about the results.

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