Everything you need to know about blood tests including why and how they are done, with links to other useful resources.
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:
- avoid eating or drinking anything, apart from water (fasting) – see can I eat and drink before having a blood test?
- stop taking a certain medication – see can I take medication before having a blood test?
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.
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.
A description of some widely used blood tests. A full blood count (FBC) is probably the most widely used blood test.
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.
International normalised ratio (INR) test
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.
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:
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
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:
- 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:
- haemophilia – a condition that affects the blood’s ability to clot (thicken)
- cystic fibrosis – a condition that causes a build-up of sticky mucus in the lungs
- spinal muscular atrophy – a condition where there is muscle weakness and progressive loss of movement
- sickle cell anaemia – a condition that causes a shortage of normal red blood cells
- polycystic kidney disease – a condition that causes cysts to develop inside the kidneys
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 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.
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:
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.
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.