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).
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 tourniquet will be released, and 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 GP. 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.
Different types of blood tests
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.
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.
This involves taking a small sample of blood from a vein in your arm and from 1 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.
At least 2 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 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 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 3 months
Blood glucose test kits may be available to use at home. These only require a small "pin prick" of blood for testing.
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.
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:
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:
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.
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. Read more about monitoring your anticoagulant dose.
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.
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.
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.
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:
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,
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.
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).
Blood Test Results (ana)
Blood test results explained
A blood test – sometimes referred to as a blood panel – is a laboratory examination of a blood sample used to check for a variety of things, including the functioning of certain organs (such as the liver, kidneys, thyroid and heart), infections and certain genetic disorders, as well as to assess an individual’s general health.
After the sample has been analyzed in the lab and the results compiled, a blood test report will in most cases be supplied to the testee. The report details the various components in the blood and at what level they are present. For those from non-medical backgrounds, the reports provided following blood tests can be complex and difficult to decipher.
Blood test abbreviations
Blood test results generally use the metric system of measurement and various abbreviations,
cmm: cells per cubic millimeter
fL (femtoliter): fraction of one-millionth of a liter
g/dL: grams per deciliter
IU/L: international units per liter
mEq/L: milliequivalent per liter
mg/dL: milligrams per deciliter
mmol/L: millimoles per liter
ng/mL: nanograms per milliliter
pg (picograms): one-trillionth of a gram
Blood test results components
A blood test is typically composed of three main tests: a complete blood count, a metabolic panel and a lipid panel. 3 Each test for different things, which can be understood through a detailed analysis of the results.
Confusingly, it is likely that the results of the three tests will not be differentiated from each other and, instead, will be listed under one large column, often labelled “Test Name”. Within each are various sub-tests, which altogether give a broad picture of an individual’s health.
Complete blood count (CBC)
The complete blood count (CBC) concentrates on the three types of blood cells: white blood cells (WBCs), red blood cells (RBCs) and platelets. By measuring the volume of blood cells, the CBC allows a doctor to evaluate an individual’s overall health, as well as check for underlying conditions such as leukemia and anemia.
The subtests within the CBC are:
White blood cell (WBC) count
Also known as leukocytes, white blood cells are a major component of the body’s immune system. A high white blood cell count can indicate the presence of infection, while a low count can point towards various conditions, including HIV/AIDS and lupus.
Differential white blood cell count
The lab tests the five main components of white blood cells and their proportion to each other. If the components are out of balance, this could indicate an infection, as well as a variety of medical conditions. Healthy proportions for each are:
Neutrophils: 40 to 60 percent of the total
Lymphocytes: 20 to 40 percent
Monocytes: 2 to 8 percent
Eosinophiles: 1 to 4 percent
Basophils: 0.5 to 1 percent
Red blood cell (RBC) count
Red blood cells (RBCs) carry oxygen to tissues throughout the body, making them important to its healthy functioning. A red blood cell count estimates the volume of RBCs within an individual – if the results show a count above or below normal levels this can indicate various medical conditions to a doctor. However, this form of testing is unable to pinpoint the root causes of any irregularities, meaning, if this is the case, further tests will be necessary.
Hematocrit (Hct) test
Tests what proportion of the blood is made up of RBCs. It is useful in diagnosing anemia, among other medical conditions.
Hemoglobin (Hgb) test
Hemoglobin is a protein contained within red RBCs that sends oxygen from the lungs to the body’s tissues. The hemoglobin test is also useful in diagnosing anemia, with many practitioners preferring this test over the hematocrit test.
Mean corpuscular volume (MCV) test
The average volume of RBCs, or the space each red blood cell fills, is measured through this test. Results outside of the normal range can be a sign of anemia or chronic fatigue syndrome, among other medical conditions.
Mean corpuscular hemoglobin (MCH) test
Red cell distribution width (RDW or RCDW) test
Tests the distribution of RBCs, not their actual size. Levels outside of the normal range can indicate conditions such as anemia, malnutrition and liver disease.
Platelets are small cells that help the blood to clot. This test measures the amount of platelets present in the blood. If testing highlights a high count, this can indicate anemia, cancer or infection, while a low count can prevent wounds from healing and result in severe bleeding.
Mean platelet volume (MPV)
Comprehensive metabolic panel (CMP)
The comprehensive metabolic panel test, also known as a chemistry panel, measures the body’s glucose levels, fluid and electrolyte balance, as well as liver and kidney function. 10 It consists of a number of sub-tests:
Alanine aminotransferase (ALT) test
Alanine aminotransferase (ALT) is an enzyme mostly produced by liver cells. 11 High levels can be an indication of liver damage.
Albumin is a protein produced by the liver. Its volume within the organ can be measured via this test. Abnormal levels can be caused by liver or kidney problems.
Total protein test
The lab tests the ratio of two types of proteins: albumin and globulin. Low protein levels can indicate various conditions, including liver and kidney disorders and malnutrition, while high levels can be a sign of inflammation, infection or bone marrow disorder.
Alkaline phosphatase test
Alkaline phosphatase is an enzyme typically produced in liver and bone cells. Results outside of the normal levels can signal liver damage and bone problems such as rickets or bone tumors.
Aspartate aminotransferase test
Aspartate aminotransferase is an enzyme usually found in RBCs and muscle tissue, as well as the heart, pancreas, liver and kidneys. This test measures the levels of this enzyme in the body, with results above the healthy range indicating a variety of conditions, including some types of cancer, as well as liver, heart or kidney damage.
Blood urea nitrogen (BUN) test
This test measures the volume of nitrogen in the blood. High levels can be caused by kidney damage or disease, while low levels may be a sign of malnutrition or severe liver damage.
This test measures the levels of calcium in the blood. If testing indicates low levels, this can indicate cancer, hyperparathyroidism, tuberculosis and other conditions, while high levels can indicate conditions including malnutrition, rickets and hypoparathyroidism.
This test measures the body’s chloride levels. An increased level of chloride can indicate dehydration as well as kidney disorders and adrenal gland dysfunction.
Creatinine is a chemical waste molecule that is important for creating muscle energy. Increased levels of creatinine can be a sign of kidney dysfunction.
Fasting blood sugar test
Blood sugar levels are easily affected by recent food or drink intake. The fasting blood sugar test is therefore done after a minimum of six hours of fasting. Abnormal results can indicate diabetes, among other medical conditions.
The lab tests the amount of phosphorus in the blood. Elevated levels can indicate problems with the kidneys and parathyroid glands, and they may be a sign of malnutrition or alcohol abuse.
Potassium aids the communication between nerves and muscles, regulates the heart and maintains muscle function. Diuretics (a substance or medication used to increase urination) can cause potassium levels to fall.
Sodium is a mineral that aids nerve impulses and muscle contractions, as well as balancing water levels. Irregularities are a possible indication of dehydration, adrenal gland disorders, corticosteroids, and kidney or liver disorders.
The lipid panel consists of various tests used to measure the different types of triglycerides (fats) and cholesterol in the blood.
Total cholesterol test
This test measures the overall levels of LDL (bad) and HDL (good) cholesterol in the blood.
Tests for triglycerides, a fat found in the blood. Irregularities are a possible risk factor for heart disease and other medical conditions.
HDL cholesterol test
HDL cholesterol, also known as high-density lipoprotein (or good cholesterol), is useful in protecting against heart disease. Low levels can increase the risk of heart problems.
LDL cholesterol test
LDL cholesterol, also known as low-density lipoprotein (or bad cholesterol), is linked to heart disease and clogged arteries.