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Blood tests





NHS Choices Syndication


Blood tests

Introduction

A blood test is taking a sample of blood for testing in a laboratory. Blood tests have a wide range of uses and are one of the most common types of medical test.

For example, a blood test can be used to:

  • assess your general state of health
  • confirm the presence of a bacterial or viral infection
  • see how well certain organs, such as the liver and kidneys, are functioning
  • screen for certain genetic conditions such as cystic fibrosis or spinal muscular atrophy

Read about some common types of blood test.

Most blood tests only take a few minutes to complete and are carried out at your GP surgery or local hospital.

Preparing for a blood test

The healthcare professional who arranges your blood test will tell you whether there are any specific instructions you need to follow before your test.

For example, depending on the type of blood test, you may be asked to:

What happens during a blood test?

A blood test usually involves taking a blood sample from a blood vessel in your arm.

The arm is a convenient part of the body to use because it can be easily uncovered. The usual place for a sample to be taken from is the inside of the elbow or wrist, where the veins are relatively close to the surface.

Blood samples from children are usually taken from the back of the hand. The child’s hand will be anaesthetised (numbed) with a special cream before the sample is taken.

A tight band (tourniquet) is usually put around your upper arm. This squeezes the arm, temporarily slowing down the flow of blood out of the arm, and causing the vein to swell with blood. This makes it easier for a blood sample to be taken.

Before taking the sample, the doctor or nurse may need to clean the area with an antiseptic wipe.

A needle attached to a syringe or to a special blood-collecting container is pushed into the vein. The syringe is used to draw out a sample of your blood. You may feel a slight pricking sensation as the needle goes in, but it should not be painful. If you do not like needles and injections, tell the person who is taking the sample so they can make you more comfortable. If you feel faint, lie down.

When the sample has been taken, the needle will be removed. Pressure is applied to the tiny break in the skin for a few minutes using a cotton-wool pad to stop the bleeding and to prevent bruising. A plaster may then be put on the small wound to keep it clean and prevent infection.

After the test

Only a small amount of blood is taken during the test so you shouldn’t feel any significant after-effects.

However, some people do feel dizzy and faint during and after the test. If this happens to you, tell the person carrying out the test so they can help you feel more comfortable.

After a blood test, you may have a small bruised area on your skin where the needle went in. Occasionally, a larger area of bruising may appear. This can be because there was a lack of pressure at the site of the jab or the blood vessel was damaged by the needle.

Bruises can be painful but are usually harmless. However, tell your GP if you frequently get bruises after having a blood test.

Results

After the blood sample has been taken, it will be put into a bottle and labelled with your name. It will then be sent to a laboratory where it will be examined under a microscope or tested with chemicals, depending on what’s being checked. The results are sent back to the hospital or to your GP, and you will be told when and how you will be given them.

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

Published Date
2014-09-03 15:56:42Z
Last Review Date
2014-03-25 00:00:00Z
Next Review Date
2016-03-25 00:00:00Z
Classification
Blood,Blood tests,Immune system




Blood tests – Health A-Z – NHS Choices






























































Blood tests 

Introduction 

Blood

Blood is pumped around the body by the heart. It supplies oxygen to the body’s organs, muscles and tissues, and removes carbon dioxide.

Blood is made up of:

  • plasma, which is a mix of water and chemicals such as proteins, glucose and salt
  • red blood cells, which carry oxygen from the lungs and transport it around the body
  • white blood cells, which form part of the body’s immune system and help defend the body against infection
  • platelets, which are cells that help the blood to clot (thicken) when you cut yourself

Donating blood

Doctors rely on blood donations to carry out life-saving treatments every day. Find out how you can help

A blood test is taking a sample of blood for testing in a laboratory. Blood tests have a wide range of uses and are one of the most common types of medical test.

For example, a blood test can be used to:

  • assess your general state of health
  • confirm the presence of a bacterial or viral infection
  • see how well certain organs, such as the liver and kidneys, are functioning
  • screen for certain genetic conditions such as cystic fibrosis or spinal muscular atrophy

Read about some common types of blood test.

Most blood tests only take a few minutes to complete and are carried out at your GP surgery or local hospital.

Preparing for a blood test

The healthcare professional who arranges your blood test will tell you whether there are any specific instructions you need to follow before your test.

For example, depending on the type of blood test, you may be asked to:

What happens during a blood test?

A blood test usually involves taking a blood sample from a blood vessel in your arm.

The arm is a convenient part of the body to use because it can be easily uncovered. The usual place for a sample to be taken from is the inside of the elbow or wrist, where the veins are relatively close to the surface.

Blood samples from children are usually taken from the back of the hand. The child’s hand will be anaesthetised (numbed) with a special cream before the sample is taken.

A tight band (tourniquet) is usually put around your upper arm. This squeezes the arm, temporarily slowing down the flow of blood out of the arm, and causing the vein to swell with blood. This makes it easier for a blood sample to be taken.

Before taking the sample, the doctor or nurse may need to clean the area with an antiseptic wipe.

A needle attached to a syringe or to a special blood-collecting container is pushed into the vein. The syringe is used to draw out a sample of your blood. You may feel a slight pricking sensation as the needle goes in, but it should not be painful. If you do not like needles and injections, tell the person who is taking the sample so they can make you more comfortable. If you feel faint, lie down.

When the sample has been taken, the needle will be removed. Pressure is applied to the tiny break in the skin for a few minutes using a cotton-wool pad to stop the bleeding and to prevent bruising. A plaster may then be put on the small wound to keep it clean and prevent infection.

After the test

Only a small amount of blood is taken during the test so you shouldn’t feel any significant after-effects.

However, some people do feel dizzy and faint during and after the test. If this happens to you, tell the person carrying out the test so they can help you feel more comfortable.

After a blood test, you may have a small bruised area on your skin where the needle went in. Occasionally, a larger area of bruising may appear. This can be because there was a lack of pressure at the site of the jab or the blood vessel was damaged by the needle.

Bruises can be painful but are usually harmless. However, tell your GP if you frequently get bruises after having a blood test.

Results

After the blood sample has been taken, it will be put into a bottle and labelled with your name. It will then be sent to a laboratory where it will be examined under a microscope or tested with chemicals, depending on what’s being checked. The results are sent back to the hospital or to your GP, and you will be told when and how you will be given them.

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

Page last reviewed: 25/03/2014

Next review due: 25/03/2016

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Comments

The 7 comments posted are personal views. Any information they give has not been checked and may not be accurate.

BLVD said on 04 September 2014

I think I have a B12 deficiency. I am 66, vegan, take h2 blockers twice a day and it got worse 2 1/2 years ago when I was given nitrous oxide for a broken compacted radius break. I have had macrocytosis but not the most recent test, also high folate result and since taking cyanocobalamin I have a blood serum test of 1207. I am tired, depressed and sometimes experience little electric feelings, sore tongue, cracked corners of mouth, feeling shaky and psychiatric problems (panic attacks) my doctor says because my result is so high I can’t have a B12 deficiency.

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AJON said on 18 July 2014

For many years I have managed to avoid blood tests, through feeling healthy, then recently, after my GP acted concerned after high blood pressure, a blood test was arranged with Practice Nurse, where I experienced same problem as previously, (by being a blood donor years ago.) The veins in my arm are very difficult to find! Now only a few day later, before my test results, I have been called back for further blood test, I have not been told the reason! But I have to go through same thing again! Why can’t NHS take blood from hand, under the circumstances, as in children, where the veins are more prominent! Am I able to request this? Also, I hate needles!

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TimRed said on 06 April 2014

I have three different blood test forms from various ares of the medical profession. Can I use these at one blood test or do I need three different appointments?

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AquaBabe said on 29 March 2014

It’s now 24 hours after my blood test and my arm is very painful when I bend my elbow. I have had lots of blood tests and donated blood but never had pain afterwards. It would help on your site if you stated how long the pain could go on for.

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dijohn said on 12 January 2014

I think it would help if on your site you could give

Information on what you need to do for a starving blood test. Patiants dont always take instrutions in when told in surgury
Diana Noble

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ethna ellen said on 13 February 2013

There is no mention of the fact that you may faint because you havent eaten. I have no fear of blood or needles, but faint each time! also, have painful arm, armpit, and feel weak 24hrs later.

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michaelcanon said on 27 April 2012

next to last sentence on this page (blood test) should read ‘but are not usually harmful…the text at present says ‘harmless !

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 /conditions/articles/blood-tests/what-it-is-used-for

Blood tests

Widely used blood tests

Blood tests can be used in a number of ways, such as helping to diagnose a condition, assessing the health of certain organs or screening for some genetic conditions.

This page describes some of the most widely used blood tests.

Full blood count (FBC)

During an FBC, a small sample of blood will be taken from a vein in your arm. The amount of different types of blood cells in the sample will be measured.

On its own, an FBC cannot usually provide a definitive diagnosis of a condition, but it can provide important “clues” about possible problems with your health, such as:

  • low haemoglobin – indicates anaemia, which has a number of possible causes, including internal bleeding or a poor diet
  • high haemoglobin – which may be caused by an underlying lung disease or problems with the bone marrow
  • a low white blood cell count – which may be genetic and of no significance, but could also be caused by problems with your bone marrow, a viral infection or more rarely, cancer of the bone marrow
  • a high white blood cell count – which usually suggests you have an infection somewhere in your body or, rarely, could be a sign of leukaemia
  • a low platelet count – which may be caused by a viral infection or an autoimmune condition (where the immune system attacks healthy tissue)
  • a high platelet count – which may be caused by inflammatory conditions, infection or a problem with the bone marrow

Erythrocyte sedimentation rate (ESR)

The test works by measuring how long it takes for red blood cells to fall to the bottom of the test tube. The quicker they fall, the more likely it is that there are high levels of inflammation.

An ESR is often used to aid diagnosis in conditions associated with inflammation such as:

Along with other tests, an ESR can be useful in confirming whether you have an infection in your body.

C-reactive protein (CRP) test

A C-reactive protein test is another blood test used to help diagnose conditions that cause inflammation.

CRP is produced by the liver and if there is a higher concentration of CRP than usual, there is inflammation in your body.

Coagulation tests

A coagulation test may be used to measure how fast your blood clots. This is used to check for bleeding disorders, such as haemophilia or von Willebrand disease.

International normalised ratio (INR) test

The international normalised ratio (INR) is used to monitor your dose of anticoagulants, such as warfarin.

During the test, a sample of your blood is taken and a chemical is added to it. The chemical starts a chain of reactions that should make the blood in the sample clot (thicken).

During the clotting process, a protein in the blood (prothrombin) turns into an enzyme called thrombin. The time that it takes the prothrombin to turn into thrombin is called the prothrombin time (“pro-time”, or PT). This is measured in seconds.

Your PT is compared with the PT of someone who is not taking warfarin, which gives your INR.

Read more detailed information about how anticoagulant medication is monitored.

Electrolyte test

Electrolytes are minerals found in the body. They have several functions, including:

  • helping to move nutrients into cells (and waste products out of them)
  • helping to maintain a healthy water balance in your body
  • helping to stabilise levels of acid and alkali in your body

There are three main electrolytes that can be measured with an electrolyte test:

  • sodium
  • potassium
  • chloride

Raised or lowered levels of any of these electrolytes can have various possible causes.

  • A raised sodium level (hypernatremia) could be the result of dehydration, uncontrolled diabetes or persistent diarrhoea.
  • A low sodium level (hyponatremia) is usually caused by certain types of medication, such as diuretics. Rarely, it could be due to a condition such as diabetes insipidus.
  • A raised potassium level (hyperkalemia) could be the results of kidney failure. Certain medications can raise potassium levels, for example ACE inhibitors, which are used to treat heart failure and high blood pressure.
  • A low potassium level (hypokalemia) could be the result of heavy sweating or persistent vomiting or diarrhoea. It can also be caused by certain medications.

Blood glucose (blood sugar) test

Most people with diabetes will need regular blood glucose tests as reducing the glucose levels is an important part of the treatment of diabetes.

This is because if the blood sugar levels become too high, a range of serious complications, such as kidney disease or nerve damage, may occur.

Blood glucose test kits may be available to use at home. These only require a small “pin prick” of blood for testing.

People with type 2 diabetes usually don’t need to check their sugar at home, it will be tested every three-to-six months at your GP surgery or hospital. The test shows the average blood sugar level over the past three months.

Some types of blood glucose test require you not to eat anything for several hours before the test. Your GP or diabetes care team can tell you whether this is the case.

Thyroid function test

If your GP suspects you have may an overactive or underactive thyroid, they will take a sample of your blood and test it for levels of:

  • thyroid-stimulating hormone (TSH)
  • thyroxine and triiodothyronine (the thyroid hormones)

If you have lower or higher-than-average levels of these hormones, it could mean you have a thyroid condition, or are at risk of developing one in the future.

Read more detailed information about:

Enzyme-linked immunosorbent assay (ELISA) test

If you have a viral or a bacterial infection, such as HIV, or you have developed an allergy, your immune system will produce specific antibodies in response to the infection or allergy.

The ELISA test takes a small blood sample and checks to see if it contains the associated antibody.

Blood tests are particularly useful when you are at risk of an extreme reaction or when a rare allergen is suspected.

Blood gases test

A blood gases sample is taken from an artery, usually at the wrist. It’s likely to be painful and the test will always be carried out in hospital.

A blood gas test is used to check two things:

  • the balance of oxygen and carbon dioxide in your blood
  • the balance of acid and alkali in your blood (the pH balance)

An imbalance in either of these can be caused by:

  • problems with your respiratory system
  • problems with your metabolism (the chemical reactions that are used by the body to break down food into energy)

Respiratory causes of an imbalance could be:

Metabolic causes of an imbalance could be:

  • diabetes
  • kidney failure
  • persistent vomiting

Genetic testing and screening

Genetic testing involves extracting a sample of DNA from your blood, then searching the sample for the suspected genetic mutation.

Genetic conditions that can be diagnosed this way include:

Genetic screening may be offered to people who are thought to be at risk of developing a genetic condition.

For example, if your brother or sister developed a genetic condition in later life, such as Huntington’s disease, you may want to find out whether there is a risk that you could also develop the condition.

Read more detailed information about genetic testing.

Chromosome testing (karotyping)

Chromosome testing involves taking a blood sample and examining one of the blood cells under a powerful microscope. This allows the person who is carrying out the test to examine the chromosomes directly.

Chromosomes are coils of DNA found in every cell. By counting the chromosomes (each cell should have 23 pairs) and by checking their shape, it may be possible to detect genetic abnormalities.

Chromosome testing is often used:

  • to test children who have physical or developmental problems that have no apparent cause
  • for couples who have experienced repeated miscarriages (usually three or more in a row)

Blood typing

Blood typing is used before:

This is because it’s important that anyone who receives blood is given blood that matches their blood group.

If you were given blood that did not match your blood group, your immune system may attack the red blood cells, which could lead to potentially life-threatening complications.

Blood typing is also used during pregnancy as there is a small risk that the unborn child may have a different blood group from the mother. This could lead to the mother’s immune system attacking the baby’s red blood cells, known as rhesus disease.

If testing reveals that there is a risk of rhesus disease developing, extra precautions can be taken to safeguard the health of your baby. For example, a blood transfusion can be given to the baby when it is still in the womb to increase their number of red blood cells.

Blood cholesterol test

Cholesterol is a fatty substance known as a lipid. It is mostly created by the liver from the fatty foods in your diet and is vital for the normal functioning of the body.

Having too many lipids in your blood (hyperlipidemia) can have a serious effect on your health because it increases your risk of having a heart attack or stroke.

Blood cholesterol testing is usually recommended if you are at an increased risk of developing cardiovascular disease (CVD). A cardiovascular disease, such as a stroke or heart attack, affects the normal flow of blood through the body.

Things that increase your risk of CVD include:

  • being over 40 years old
  • being obese
  • being a smoker
  • being male
  • having high blood pressure (hypertension)

Blood cholesterol levels are measured with a simple blood test. Before having the test, you may be asked not to eat for 12 hours (which usually includes when you’re asleep). This will ensure that all food is completely digested and won’t affect the outcome of the test.

Your GP or practice nurse can carry out the blood test and will take a blood sample, either using a needle and a syringe or by pricking your finger.

Read more information about diagnosing high cholesterol.

Liver function test

When the liver is damaged, it releases enzymes into the blood and levels of proteins that the liver produces begin to drop.

By measuring the levels of these enzymes and proteins, it’s possible to build up a picture of how well the liver is functioning.

This can help to diagnose certain liver conditions, including:

Blood culture

A blood culture involves taking a small sample of blood from a vein in your arm and from another part of your body.

Both samples are introduced to nutrients designed to encourage the growth of bacteria (a process known as culturing). If there are traces of bacteria in your blood, culturing should highlight this.

Two blood samples are needed in case one is accidentally contaminated by the bacteria that live on your skin.

Amylase test

An amylase test is often used with another blood test, known as a lipase test, to help diagnose or monitor conditions that affect the pancreas.

Your blood sample is tested for levels of amylase and lipase. An increased level of both could lead to a diagnosis of:

Prostate-specific antigen (PSA) test

Prostate-specific antigen (PSA) is made by the prostate gland. Some of it will leak into your blood, and the amount depends on your age and the health of your prostate.

A raised PSA level in your blood may show that you have a problem with your prostate, such as:

Read more detailed information about the reasons for having a prostate-specific antigen (PSA) test.

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Published Date
2014-03-27 11:19:46Z
Last Review Date
2014-03-25 00:00:00Z
Next Review Date
2016-03-25 00:00:00Z
Classification
Anaemia,Blood,Blood tests,Blood transfusion,Cholesterol test,Coronary heart disease,Electrolytes,Healthy eating,Heart and vascular diseases,Hypertension,Infection tests,Kidney disease,Liver,Liver diseases,Liver function tests,Red blood count,Rhesus factor disease


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