Blood Tests

This article is based on a number of articles I have read and my own experience. However, since I am not a physician, do not claim it to be authentic. It is given for general guidance.
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Introduction

Blood is pumped around the body by the heart. It supplies oxygen to the body’s organs, muscles and tissues, and removes carbon dioxide. Blood is made up of several different kinds of cells and other compounds, including various salts and certain proteins. The liquid portion of the blood is called plasma.
Blood is made up of:
•    plasma, which is a mix of water and chemicals such as proteins, glucose and salt
•    red blood cells, which carry oxygen from the lungs and transport it around the body
•    white blood cells, which form part of the body’s immune system and help defend the body against infection
•    platelets, which are cells that help the blood to clot (thicken) when you cut yourself

Blood Tests

Blood tests can reveal valuable information about the cause of a disease and its symptoms (diagnosis) and are also valuable in monitoring the effects of treatment. 
A blood test is when a sample of blood is taken for testing in a laboratory. 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
•    confirm the presence of a bacterial or viral infection
•    see how well certain organs, such as the liver and kidneys, are functioning
•    screen for certain genetic conditions such as cystic fibrosis.  

What happens during a blood test?

Most blood tests are carried out at a Pathology Lab or a hospital by trained personnel. A test usually involves placing a needle attached to a syringe into one of the blood vessels in the inside of elbow. The arm is a convenient part of the body to use because it can be easily uncovered. A tight band (tourniquet) is usually put around the upper arm. This squeezes the arm, temporarily slowing down the flow of blood out of the arm, and causing the vein to swell with blood. This makes it easier for a blood sample to be taken. Blood samples from children may be taken from the back of the hand. A sample of blood is taken and the needle is removed. A small cotton-wool pad is pressed on the site of the blood removal, which stops any bleeding. Most blood tests take just a few minutes to complete.
If blood is taken from an artery, it is usually extracted from the wrist where there is an artery that is very close to the skin. This may be slightly uncomfortable, as the artery wall has more pain nerves in it than the vein wall. After taking blood from an artery it may be necessary to hold a ball of cotton wool on the place where the injection was made for about five minutes to stop any bleeding.
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 happens, or one is afraid of this possibility, the person carrying out the test should be told.

After the test

After the blood sample has been taken, it will be put into a bottle and labelled with the patient's name. It will then be examined in a laboratory under a microscope or tested with chemicals, depending on what's being checked. When blood clots outside the body, the blood cells and some of the proteins become solid. The remaining liquid is called serum, which can be used in chemical tests and in tests to find out how the immune system fights diseases.
In Culture tests the infectious organisms that cause an illness are grown in a conducive environment to see exactly what they are through a microscope.
Sometimes, receiving results can be stressful and upsetting. If one is worried about the outcome of a test, he may choose to take a trusted friend or relative with him.

What is examined in the blood test?

The tests include measurements of the levels of the blood cells (red blood cells, white blood cells and platelets) and a blood smear. A blood smear is a film of blood placed on a slide to allow doctors to look at the individual cells under a microscope. Some widely used blood tests are described below.

Full blood count (FBC)

A full blood count (FBC) is probably the most widely used blood test. It is used to assess general state of health and to screen for certain conditions, such as anaemia. During an FBC, the amount of different types of blood cells in a sample will be measured. On its own, an FBC cannot usually provide a definitive diagnosis of a condition, but it can provide important "clues" about possible problems with health.

Red blood cells

One of the most important red blood cell tests is used to find out how much haemoglobin there is in the blood. Haemoglobin carries oxygen around your body. This is called the haemoglobin concentration or level. Another important test, the mean corpuscular volume or MCV test, measures the size of the red blood cells.
If a person suffers from anaemia their haemoglobin level will always be less than normal. But the size of the red blood cells depends on the type of anaemia one may have.
A haematocrit test measures the total volume that red blood cells take up in the blood. If the red blood cells are pale, it can be a sign of iron deficiency anaemia. If they have a strange shape, it may be because of sickle cell anaemia or pernicious anaemia.
•    Low haemoglobin indicates anaemia, which has a number of possible causes, including internal bleeding or a poor diet.
•    High haemoglobin may be due to an underlying lung disease or problems with the bone marrow.

White blood cells (WBC)

The doctor counts the total number of white blood cells and works out how many different types of white blood cells the patient has. This is called the differential WBC count. The number of white blood cells may go up and this may be because of a bacterial infection, bleeding or a burn. More rarely the cause of a raised white count is due to leukaemia, cancer or malaria. A person may lose white blood cells because they have autoimmune problems - this is where the antibodies that should fight diseases attack the body instead. Other reasons for loss of white blood cells include viral infections. More rarely, this can be a side effect of certain kinds of medication. Doctors keep an eye on white blood cells to work out how a disease is changing. By monitoring the blood count in this way they can alter the patient's treatment as necessary.
•    A low white blood cell count may be due to problems with your bone marrow, a viral infection or more rarely, cancer of the bone marrow. However, a low white blood count can also be genetic and of no significance. 
•    A high white blood cell count usually suggests that you have an infection somewhere in your body. Rarely, this could be a sign of leukaemia.

Platelets

Platelets are very small cells in the blood that clump together at sites of injury to blood vessels. They form the basis of the blood clot that would form if you cut yourself. Low numbers of platelets can make a person vulnerable to bleeding, sometimes even without injury occurring. Causes of low platelet counts include autoimmune diseases where one's body produces an antibody to his own platelets, chemotherapy, leukaemia, viral infections and some medicines.
High numbers of platelets make a person more vulnerable to blood clots. High platelet counts are found in conditions involving the bone marrow such as leukaemia and cancer.
•    A low platelet count may be due to a viral infection or an autoimmune condition (where the immune system attacks healthy tissue).
•    A high platelet count may be due to inflammatory conditions, infection or a problem with the bone marrow.

Electrolyte test

An electrolyte test is used to measure the levels of electrolytes in your blood. This is sometimes known as your electrolyte balance. Electrolytes are minerals that are found in the body. They have several functions, including:
•    helping to move nutrients into cells (and waste products out of them)
•    helping to maintain a healthy water balance in your body
•    helping to stabilise levels of acid and alkali in your body
There are three main electrolytes that can be measured with an electrolyte test:
•    sodium
•    potassium
•    chloride
Raised or lowered levels of any of these electrolytes can have various possible causes.
•    A raised sodium level (hypernatremia) could be the result of dehydration, uncontrolled diabetes or persistent diarrhoea.
•    A low sodium level (hyponatremia) is usually due to certain types of medication, such as diuretics. Rarely, it could be due to a condition such as diabetes insipidus.
•    A raised potassium level (hyperkalemia) could be the results of kidney failure. Certain medications can raise potassium, for example ACE inhibitors, which are used to treat heart failure and high blood pressure.
•    A low potassium level (hypokalemia) could be the result of heavy sweating or persistent vomiting or diarrhoea. It can also be caused by certain medications.

Erythrocyte sedimentation rate (ESR)

An erythrocyte sedimentation rate (ESR) test is a blood test that is used to check whether there is inflammation in the body.
The test works by measuring how long it takes for red blood cells to fall to the bottom of the test tube. The quicker they fall the more likely it is that there are high levels of inflammation. An ESR is often used to aid diagnosis in conditions associated with inflammation such as arthritis, Crohn’s disease, temporal arteritis and polymyalgia rheumatica. Along with other tests, an ESR can be useful in confirming whether one has an infection in your body.

Blood glucose (blood sugar) test

A blood glucose test is used to help diagnose diabetes and to monitor the health of people who have been diagnosed with diabetes. Diabetes develops either because the body cannot produce enough insulin or because the insulin doesn't work in the right way. Insulin is a hormone that the body uses to convert glucose (sugar) into energy. People with diabetes often have high levels of glucose in their blood. Reducing the glucose levels is an important part of the treatment of diabetes. This is because if the blood sugar levels become too high, a range of serious complications, such as kidney disease or nerve damage, may occur. Therefore, most people with diabetes will need regular blood glucose tests. Blood glucose test kits may be available to use at home. These only require a small "pin prick" of blood for testing. People with type 2 diabetes usually don't need to check their sugar at home, it will be tested every three-to-six months at a laboratory or hospital. This HbA1C test shows the average blood sugar level over the past three months. Blood glucose test are usually carried out with Empty stomach or 2 hours after breakfast or lunch.

Blood typing

A blood-typing test is used to identify blood group. The blood group is determined by two specialised proteins, known as antigens, which are found on the surface of red blood cells. Blood typing is used before a blood transfusion is given (or before providing blood for donation). This is because it's important that anyone who receives blood is given blood that matches his blood group. If a person is given blood that did not match his blood group, his 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 is a small risk that the unborn child may have a different blood group from the mother. This could lead to the mother's immune system attacking the baby’s red blood cells. This is known rhesus disease. If testing reveals that there is a risk of rhesus disease developing, extra precautions can be taken to safeguard the health of the baby. For example, a blood transfusion can be given to the baby when it is still in the womb to increase their number of red blood cells.

Blood gas test

A blood gases sample is taken from an artery, usually at the wrist. It's likely to be painful and the test will alway be carried out in a hospital.
A blood gas test is used to check two things:
•    the balance of oxygen and carbon dioxide in blood
•    the balance of acid and alkali in blood (the pH balance)
An imbalance in either of these can be caused by:
•    problems with respiratory system
•    problems with metabolism (the chemical reactions that are used by the body to break down food into energy)
Respiratory causes of an imbalance could be:
•    pneumonia
•    chronic obstructive pulmonary disorder (COPD)
•    hyperventilation
Metabolic causes of an imbalance could be:
•    diabetes
•    kidney failure
•    persistent vomiting

Blood cholesterol test

Cholesterol is a fatty substance known as a lipid. It is mostly created by the liver from the fatty foods in the diet and is vital for the normal functioning of the body. Having too many lipids in blood (hyperlipidemia) can have a serious effect on health because it increases risk of having a heart attack or stroke. Blood cholesterol testing is usually recommended when there is a risk of developing cardiovascular disease (CVD), such as a stroke or heart attack, affecting the normal flow of blood through the body. Blood cholesterol levels are measured with a simple blood test. Before having the test, one may be asked not to eat for 12 hours. This will ensure that all food is completely digested and won't affect the outcome of the test.

Liver function test

A liver function test is a type of blood test that is used to help diagnose certain liver conditions, such as:
•    hepatitis (infection of the liver)
•    cirrhosis (scarring of the liver)
•    alcoholic liver disease (liver damage and related loss of function which is caused by excessive alcohol consumption)
When the liver is damaged, it releases enzymes into the blood and levels of proteins that the liver produces 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.

Blood culture

A blood culture is a test to check whether there is a bacterial infection of the blood (septicaemia). Septicaemia is potentially very dangerous because it can trigger a massive drop in blood pressure. This is known as septic shock and it can be fatal. A blood culture involves taking a small sample of blood from a vein in the arm and from another part of body. Both samples are introduced to nutrients designed to encourage the growth of bacteria (a process known as culturing). If there are traces of bacteria in the blood, culturing should highlight this. Two blood samples are needed in case one is accidentally contaminated by the bacteria that live on skin.

Blood coagulation examinations

More tests will be needed if a patient is found to be suffering from a blood coagulation disorder so that either their blood doesn't clot properly, or if it clots too well. When a vein is damaged, usually a little blood clot will form on the inside. This clot is made of blood platelets and proteins from the blood plasma (called the coagulation factors). A person will bleed more than normal if he has a low number of blood platelets, if there is a lack of coagulation factors, or if they don't work.
If the bleeding disorder is caused by problems with the coagulation factors more tests will be needed. Sometimes a coagulation disorder is passed on in the family, but it could also be due to a liver problem, as the liver makes many of the blood clotting factors. Coagulation tests will be performed regularly for people who are on blood thinning medicines such as warfarin. Doctors will change the dose of these medicines depending on the test results

ELISA test

An enzyme-linked immunosorbent assay (ELISA) test can be used to check whether one has an infection, such as HIV, or a specific allergy, such as a peanut allergy. If he has a viral or a bacterial infection, or has developed an allergy, his immune system will produce specific antibodies in response to the infection or allergy. The ELISA test takes a small blood sample and checks to see if it contains the associated antibody.

Gene test

A gene test is used when healthcare professionals suspect that a specific genetic mutation may be responsible for a person’s symptoms. The test involves extracting a sample of DNA from his blood, then searching the sample for the suspected genetic mutation. Genetic conditions that can be diagnosed this way include:
•    haemophilia: a condition that affects the blood’s ability to clot (thicken)
•    cystic fibrosis: a condition that causes a build-up of sticky mucus in the lungs
•    sickle cell anaemia: a condition that causes a shortage of normal red blood cells
•    polycystic kidney disease: a condition that causes cysts to develop inside the kidneys

Genetic screening

Genetic screening is similar to gene testing except that it is used in people who have no obvious symptoms.
Genetic screening may be offered to people who are thought to be at 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.

Chromosome testing

Chromosome testing, also known as karyotyping, is a more general test than a gene test. It is used when gene-related problems are suspected, but the healthcare professionals don't know which gene is responsible. Chromosome testing involves taking a blood sample and examining one of the blood cells under a powerful microscope. This allows the person who is carrying out the test to examine the chromosomes directly. Chromosomes are coils of DNA found in every cell. By counting the chromosomes (each cell should have 23 pairs) and by checking their shape, it may be possible to detect genetic abnormalities.
Chromosome testing is often used:
•    to test children who have physical or developmental problems that have no apparent cause
•    for couples who have experienced repeated miscarriages (usually three or more in a row)