Common blood tests

Information on some of the most widely used blood tests


Blood cholesterol test

Cholesterol is a fatty substance mostly created by the liver from the fatty foods in your diet. It's vital for the normal functioning of the body.

Having a high level of cholesterol can contribute to an increased risk of serious problems like heart attacks 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. However, this isn't always necessary.

Read about cholesterol tests

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:

Read more about blood gas tests

Blood typing

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. This 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. This 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.

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.

Find out more about blood tests during pregnancy

Read more about blood typing

Blood culture

Blood culture 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.

2 or more samples are usually needed.

Read more about blood cultures

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 8 hours
  • glucose tolerance test – where the level of glucose in your blood is checked after fasting, and again 2 hours later after being given a glucose drink
  • HbA1C test – a test done at your GP surgery 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


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, 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
Coagulation tests

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 like haemophilia or von Willebrand disease.

A type of coagulation test called the international normalised ratio (INR) is used to monitor the dose of anticoagulants, like warfarin, and check that your dose is correct.

Read more about coagulation factors and international normalised ratio

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
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's a higher concentration of CRP than usual, it's a sign of inflammation in your body.

Read more about C-reactive protein


Electrolyte test

Electrolytes are minerals found in the body, including sodium, potassium and chloride. They perform jobs like maintaining a healthy water balance in your body.

Changes in the level of electrolytes can have various possible causes, including:

Read more about electrolytes

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, like:

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

Read more about erythrocyte sedimentation rate


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
  • 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


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.


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:

Read more about liver function tests


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 like an underactive thyroid or overactive thyroid.

Read more about thyroid function tests