What is a blood test?
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
- check if you have an infection
- see how well certain organs, such as the liver and kidneys, are working
- screen for certain genetic conditions
Most blood tests only take a few minutes to complete and are carried out at your GP surgery or local hospital by a doctor, nurse or phlebotomist (a specialist in taking blood samples).
Read about some common types of blood test .
For more information about a wider range of tests, search the blood test A-Z index on Lab Tests Online UK.
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) for up to 12 hours – 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?
It's important to follow the instructions you're given, as it may affect the result of the test and mean it needs to be delayed or carried out again.
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 often taken from the back of the hand. Their skin may be numbed with a special spray or 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 and causing the vein to swell. This makes it easier for a sample to be taken.
Before taking the sample, the doctor or nurse may clean the area of skin with an antiseptic wipe.
A needle attached to a syringe or special container is inserted into the vein. The syringe is used to draw 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. If you don't like needles and blood, tell the person who is taking the sample so they can make you more comfortable.
When the sample has been taken, the needle will be removed. Pressure is applied to the skin for a few minutes using a cotton-wool pad. A plaster may be put on the small wound to keep it clean.
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 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 so they’re aware and can help you feel more comfortable.
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 the blood sample has been taken, it will be put into a bottle and labelled with your name and details. 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 doctor. Some test results will be ready the same day or a few days later, although others may not be available for a few weeks. You'll be told when your results will be ready and how you'll be given them.
Sometimes, receiving results can be stressful and upsetting. If you're 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.
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 the following commonly used blood tests:
- blood cholesterol test
- blood culture
- blood gases test
- blood glucose (blood sugar) tests
- blood typing
- cancer blood tests
- chromosome testing (karyotyping)
- coagulation tests and international normalised ratio (INR)
- C-reactive protein (CRP) test
- electrolyte test
- erythrocyte sedimentation rate (ESR)
- full blood count (FBC)
- genetic testing and screening
- liver function test
- thyroid function test
For more information about a wider range of tests, search the blood test A-Z index on Lab Tests Online UK.
Blood cholesterol test
Cholesterol is a fatty substance mostly created by the liver from the fatty foods in your diet and is vital for the normal functioning of the body.
Having a high level of cholesterol can contribute to an increased risk of serious problems such as heart attacks](yourmd:/condition/heart-attack) and [strokes .
Blood cholesterol levels can be measured with a simple blood test. You may be asked not to eat for 12 hours before the test (which usually includes when you're asleep) to ensure that all food is completely digested and won't affect the result, although this isn't always necessary.
Find out more about diagnosing high cholesterol and read about cholesterol tests on Lab Tests Online UK.
This involves taking a small sample of blood from a vein in your arm and from one or more other parts of your body.
The samples are combined with nutrients designed to encourage the growth of bacteria. This can help show whether any bacteria are present in your blood.
Two or more samples are usually needed.
Read more about blood cultures on Lab Tests Online UK.
Blood gases test
A blood gases sample is taken from an artery, usually at the wrist. It's likely to be painful and is only carried out in hospital.
A blood gas test is used to check the balance of oxygen and carbon dioxide in your blood, and the balance of acid and alkali in your blood (the pH balance).
A pH imbalance can be caused by:
- problems with your respiratory system, such as pneumonia or chronic obstructive pulmonary disease (COPD)
- problems affecting your metabolism (the chemical reactions used by the body to break down food into energy), such as diabetes, kidney failure or persistent vomiting
Read more about blood gas tests on Lab Tests Online UK.
Blood glucose (blood sugar) tests
A number of tests can be used to diagnose and monitor diabetes by checking the level of sugar (glucose) in the blood.
These include the:
- fasting glucose test – where the level of glucose in your blood is checked after fasting (not eating or drinking anything other than water) for at least eight hours
- glucose tolerance test – where the level of glucose in your blood is checked after fasting, and again two hours later after being given a glucose drink
- HbA1C test – a test done at your doctor or hospital to check your average blood sugar level over the past three months
Blood glucose test kits may be available to use at home. These only require a small "pin prick" of blood for testing.
Read more about glucose tests on Lab Tests Online UK.
This is done before donating blood or having a blood transfusion , to check what your blood group is.
If you were given blood that didn't 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's a small risk the unborn child may have a different blood group from their mother, which could lead to the mother's immune system attacking her baby's red blood cells. This is known as rhesus disease .
If you don’t already know your blood type, your blood will be tested at least once during your pregnancy to determine if there’s a risk of rhesus disease. Read more about diagnosing rhesus disease .
If testing reveals there is a risk of rhesus disease, an injection of a medicine that stops the mother's immune system attacking her baby's blood cells can be given. Read more about preventing rhesus disease .
Read more about blood typing on Lab Tests Online UK.
Cancer blood tests
A number of blood tests can be carried out to help diagnose certain cancers or check if you're at an increased risk of developing a particular type of cancer.
These include tests for:
- prostate-specific antigen (PSA) – this can help diagnose prostate cancer](yourmd:/condition/cancer-of-the-prostate) , although it can also detect other problems such as an enlarged prostate or [prostatitis
- CA125 protein – a protein called CA125 can indicate ovarian cancer , although it can also be a sign of other things such as pregnancy or pelvic inflammatory disease (PID)
- BRCA1 and BRCA2 genes – certain versions of these genes can greatly increase a woman's chance of developing breast cancer and ovarian cancer; this test may be carried out if these types of cancer run in your family
Chromosome testing (karyotyping)
This is a test to examine bundles of genetic material called chromosomes.
By counting the chromosomes (each cell should have 23 pairs) and checking their shape, it may be possible to detect genetic abnormalities.
Chromosome testing can be used:
- to help diagnose disorders of sex development (DSDs) , such as androgen insensitivity syndrome
- for couples who have experienced repeated miscarriages , to see if a chromosomal problem could be responsible
A coagulation test may be used to see if your blood clots in the normal way.
If it takes a long time for your blood to clot, it may be a sign of a bleeding disorder such as haemophilia or von Willebrand disease .
A type of coagulation test called the international normalised ratio (INR) is used to monitor the dose of anticoagulants , such as warfarin](yourmd:/condition/anticoagulants-warfarin) , and check that your dose is correct. Read more about [monitoring your anticoagulant dose .
Read more about coagulation factors and international normalised ratio on Lab Tests Online UK.
C-reactive protein (CRP) test
This is another 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, it's a sign of inflammation in your body.
Read more about C-reactive protein on Lab Tests Online UK.
Electrolytes are minerals found in the body, including sodium, potassium and chloride, that perform jobs such as maintaining a healthy water balance in your body.
Changes in the level of electrolytes can have various possible causes, including dehydration , diabetes or certain medications.
Read more about electrolytes on Lab Tests Online UK.
Erythrocyte sedimentation rate (ESR)
This test works by measuring how long it takes for red blood cells to fall to the bottom of a test tube. The quicker they fall, the more likely it is there are high levels of inflammation.
An ESR is often used to help diagnose conditions associated with inflammation, such as:
Along with other tests, an ESR can also be useful in confirming whether you have an infection.
Read more about erythrocyte sedimentation rate on Lab Tests Online UK.
Full blood count (FBC)
This is a test to check the types and numbers of cells in your blood, including red blood cells, white blood cells and platelets.
This can help give an indication of your general health, as well as provide important clues about certain health problems you may have.
For example, an FBC may detect signs of:
- iron deficiency anaemia or vitamin B12 deficiency anaemia
- infection or inflammation
- bleeding or clotting disorders
Read more about full blood count on Lab Tests Online UK.
Genetic testing and screening
This involves extracting a sample of DNA from your blood, then searching the sample for a specific genetic change (mutation).
Genetic conditions that can be diagnosed this way include:
- haemophilia – a condition that affects the blood’s ability to clot
- cystic fibrosis – a condition that causes a build-up of sticky mucus in the lungs
- spinal muscular atrophy – a condition involving 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 fluid-filled sacs called cysts to develop in the kidneys
Genetic screening can also be used to check if someone carries a particular gene that increases their 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 about genetic testing .
Liver function test
When the liver is damaged, it releases substances called enzymes into the blood and levels of proteins produced by the liver 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 hepatitis , cirrhosis (liver scarring), and alcohol-related liver disease .
Read more about liver function tests on Lab Tests Online UK.
Thyroid function test
This test is used to test your blood for levels of thyroid-stimulating hormone (TSH), and, where needed, thyroxine and triiodothyronine (thyroid hormones).
If you have low or high levels of these hormones, it could mean you have a thyroid condition such as an underactive thyroid or overactive thyroid .
Read more about thyroid function tests on Lab Tests Online UK.