What can doctors tell from a blood 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.

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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:

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 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.

Blood culture

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.

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

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 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.

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, which could lead to potentially life-threatening complications.

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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.

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

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

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 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, and check that your dose is correct.

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.

Electrolyte test

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.

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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. 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.

iron deficiency anaemia or vitamin B12 deficiency anaemia

infection or inflammation

bleeding or clotting disorders

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 hepatitis, cirrhosis (liver scarring), and alcohol-related liver disease.

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.

What can doctors tell from a blood test?

Is peeing 3 times at night normal?

Can you eat bread with kidney disease?

This page describes the following commonly used blood tests:

Blood cholesterol test

Blood culture

Two or more samples are usually needed.

Blood gases test

A pH imbalance can be caused by:

Blood glucose (blood sugar) tests

These include the:

Blood typing

What foods plug your arteries?

What triggers the prostate to enlarge?

Cancer blood tests

These include tests for:

Chromosome testing (karyotyping)

This is a test to examine bundles of genetic material called chromosomes.

Chromosome testing can be used:

Coagulation tests

C-reactive protein (CRP) test

This is another test used to help diagnose conditions that cause inflammation.

Electrolyte test

How do you know if your testosterone level is high?

What is the #1 health threat to men?

Erythrocyte sedimentation rate (ESR)

iron deficiency anaemia or vitamin B12 deficiency anaemia

infection or inflammation

bleeding or clotting disorders

Genetic testing and screening

Genetic conditions that can be diagnosed this way include:

haemophilia – a condition that affects the blood’s ability to clot

Liver function test

Thyroid function test

Read more about thyroid function tests on Lab Tests Online UK.

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