Blood tests can be used to:
- help diagnose a condition
- assess the health of certain organs
- screen for some genetic conditions
- monitor treatment
This page describes some commonly used blood tests.
Blood cholesterol test
Cholesterol is a fatty substance mostly created by the liver from the fatty foods in your diet. It is vital for the normal functioning of the body.
Having a high level of cholesterol can put you at risk of serious problems. This includes heart attacks and strokes.
A simple blood test can measure blood cholesterol levels.
The test is usually performed early in the day. You may be asked to fast for 12 hours before it. This is to make sure that all food is completely digested and won't affect the result.
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.
They use a blood gas test to check:
- the balance of oxygen and carbon dioxide in your blood
- the balance of acid and alkali in your blood (the pH balance)
Blood glucose (blood sugar) tests
A few tests can diagnose and track diabetes by checking the level of sugar (glucose) in the blood.
These include the:
- fasting glucose test – where a health professional checks the level of glucose in your blood after fasting. Fasting is when you don't eat or drink anything other than water for at least 8 hours
- glucose tolerance test – where a health professional checks the level of glucose in your blood after fasting, and again 2 hours later after you have a glucose drink
- HbA1C test – a test done at your GP surgery or hospital to check your average blood sugar level over the past 3 months
Blood glucose test kits may be available to use at home. These only need a small 'pin-prick' of blood for testing.
This is to check what your blood group is. You will need this test before donating blood or having a blood transfusion.
If you need a blood transfusion the laboratory will match your blood with donor blood.
Blood typing during pregnancy
Blood typing is also used during pregnancy. There's a small risk the unborn child may have a different blood group from their mother. This could lead to rhesus disease. Rhesus disease is when the mother's immune system attacks her baby's red blood cells.
If your blood type is rhesus negative, you will have to get an anti-D injection in a hospital. An anti-D injection stops your immune system attacking your baby's blood cells.
Cancer blood testing
Different blood tests can help to diagnose certain cancers. These can also 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. It can also detect other problems such as an enlarged prostate or prostatitis.
A protein called CA125 can show ovarian cancer. But 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 increase a woman's chance of developing:
- breast cancer
- ovarian cancer
You may get this test if these types of cancer run in your family.
Chromosome testing (karyotyping)
This is a test to examine bundles of genetic material called chromosomes.
It may be possible to detect genetic abnormalities by:
- counting the chromosomes (each cell should have 23 pairs)
- checking their shape
Chromosome testing can help:
- to diagnose disorders of sex development (DSDs), such as androgen insensitivity syndrome
- couples who have had repeated miscarriages, to see if a chromosomal problem is the cause
You may get a coagulation test 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, for example, haemophilia or Von Willebrand disease.
You may be on an anticoagulant such as warfarin. Your doctor or nurse may order a coagulation test called the international normalised ratio (INR). This monitors the dose of anticoagulant and checks that your dose is correct.
C-reactive protein (CRP) test
This is a test used to help diagnose conditions that cause inflammation.
Your liver produces CRP. If there is a higher concentration of CRP than usual, it's a sign of inflammation in your body.
Electrolytes are minerals found in the body.
They perform jobs like maintaining a healthy water balance in your body.
Changes in the level of electrolytes can have various possible causes.
- certain medications
Full blood count (FBC)
FBC is also known as complete blood count (CBC). It is a test to check the types and numbers of cells in your blood.
- red blood cells
- white blood cells
The FBC is also used to check your haemoglobin. Haemoglobin is the iron-rich protein in red blood cells that carries oxygen to the body.
This can help give an overview of your general health. It also provides important clues about certain health problems you may have.
For example, an FBC may show signs of:
- iron deficiency anaemia or vitamin B12 deficiency anaemia
- infection or inflammation
- bleeding or clotting disorders
Genetic testing and screening
For genetic testing the laboratory usually extracts a sample of DNA from your blood. They then search 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 cysts (fluid-filled sacs) in the kidneys
You can also get screened to check if you're at risk of developing a genetic condition. For example, if your sibling developed a genetic condition 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. Levels of proteins produced by the liver begin to drop.
By measuring the levels of these enzymes and proteins, you can see how well the liver is functioning.
This can help to diagnose certain liver conditions including:
- cirrhosis (liver scarring)
- alcohol-related liver disease
Thyroid function test
This tests your blood for levels of thyroid-stimulating hormone (TSH). It can also test for thyroxine and triiodothyronine (thyroid hormones).
If you have low or high levels of these hormones, it could mean you have a thyroid condition. For example, an underactive thyroid or overactive thyroid.
Content supplied by the NHS and adapted for Ireland by the HSE