Pathology Laboratory

How is a Blood Test Normally Done?

It’s important to get regular blood tests to make sure that you are healthy. How is a blood test normally done? The vein used for blood sampling is usually on the inside of your elbow or the back of your wrist. A tight band (tourniquet) is usually placed around your upper arm. This makes the vein fill with blood and makes it easier for the blood sample to be taken.

  • The vein used for blood sampling is usually on the inside of your elbow or the back of your wrist.
  • A tight band (tourniquet) is usually placed around your upper arm. This makes the vein fill with blood and makes it easier for the blood sample to be taken.
  • The skin over the vein may be cleaned with an antiseptic wipe.
  • A needle is then inserted into the vein through the clean skin, causing a sharp pricking sensation. The needle is connected either to a syringe, or directly to blood sample bottles.
  • When the required amount of blood is taken, the needle is removed. The small wound is pressed on with cotton wool for a few minutes to stop the bleeding and prevent bruising. A sticking plaster may be put on. The blood is placed in bottles. There may be a slight ache following a blood test.

Variations of Blood Withdrawing for Lab Test

Some blood tests require several samples taken over a period of time. For example, they may be done to check how you respond to something. The person who requests the blood test will write on the form which tests they want the laboratory to do.

  • Some blood tests require several samples taken over a period of time. For example, they may be done to check how you respond to something. If you require repeated samples fairly close to each other (over the following few hours or so), a doctor may insert a ‘butterfly’ needle into the vein, which can be taped to the skin. Samples of blood can then be taken without using a needle each time.
  • If only a small amount of blood is needed, then a few drops of blood can be squeezed out from a small prick in the tip of the finger or earlobe. For example, only a small amount of blood is needed for checking the blood sugar (glucose) level, using a test strip.
  • Some blood tests are taken from an artery in the wrist. For example, to measure the level of oxygen in the artery. This is usually only done in hospital in certain circumstances
  • You may be told not to eat for a time before certain tests. For example, a test of blood glucose is commonly done first thing in the morning before you have anything to eat.
  • Children who need a blood test may be given cream to put on at home before the test to numb the skin.

Are there any complications from blood taking?

Are there any complications from blood taking? Sometimes a bruise develops where the needle was inserted. This is much less likely to happen if you press over the site with cotton wool for several minutes with your arm left straight (not bent).

  • Sometimes a bruise develops where the needle was inserted. This is much less likely to happen if you press over the site with cotton wool for several minutes with your arm left straight (not bent).
  • As with any wound, an infection may develop where the needle was inserted. See your doctor if the wound site becomes red and inflamed.
  • Rarely, some people feel faint during a blood test. Tell the person doing the test if you feel faint, as you should immediately lie down to prevent fainting.

Different types of blood samples for lab testing

Blood tests look at different substances in the blood. They can provide important information about your health. The tests are most often done by taking a blood sample from a vein, a procedure known as Venipuncture. A blood test is a laboratory analysis performed on a blood sample that is usually extracted from a vein in the arm using a hypodermic needle, or via finger prick. Multiple tests for specific blood components, such as a glucose test or a cholesterol test, are often grouped together into one test.

Blood can be tested for many different things. The person who requests the blood test will write on the form which tests they want the laboratory to do. Different blood bottles are used for different tests. For example, for some tests the blood needs to clot and the test is looking for something in the serum. For some tests, the blood is added to some chemicals to prevent it from clotting. If the blood glucose is being measured, then the blood is added to a special preservative, etc. This is why you may see your blood added to blood bottles of different sizes and colours.

Blood tests are taken for many different reasons – for example,

  • Help diagnose certain conditions, or to rule them out if symptoms suggest possible conditions.
  • Monitor the activity and severity of certain conditions. For example, a blood test may help to see if a condition is responding to treatment.
  • Check the body’s functions such as liver function and kidney function when you are taking certain medicines which may affect the liver or the kidneys.
  • Check your blood group before receiving a blood transfusion
Which is the most common (important) blood test?

A CBC or Hemogram is one of the most common blood tests done. A blood sample is taken, which is prevented from clotting by using a preservative in the blood bottle. The sample is put into a machine which automatically:

  • Counts the number of red cells, white cells and platelets per millilitre of blood.
  • Measures the size of the red blood cells and calculates their average (mean) size.
  • Calculates the proportion of blood made up from red blood cells (the haematocrit).
  • Measures the amount of haemoglobin in the red blood cells.

The main abnormalities which can be detected are:

  • Anaemia: this means that you have fewer red blood cells than normal, or have less haemoglobin than normal in each red blood cell. The most common reason for an FBC to be done is to check for anaemia. There are many causes of anaemia. The average size of the red cells can give a clue as to the cause of some anaemias. (Iron is needed to make haemoglobin.) With this type of anaemia, the average size of the red blood cells is smaller than normal.
  • Too many red cells: this is called polycythaemia and can be due to various causes.
  • Too few white cells: this is called leukopenia. Depending on which type of white cell is reduced, it can be called neutropenia, lymphopenia or eosinopenia. There are various causes.
  • Too many white blood cells: this is called leukocytosis. Depending on which type of white cell is increased it is called neutrophilia, lymphocytosis, eosinophilia, monocytosis or basophilia. There are various causes – for example:
    • Various infections can cause an increase of white blood cells.
    • Certain allergies can cause an eosinophilia.
    • Leukaemia is a type of blood cancer where there are numerous abnormal cells, usually white blood cells. The type of leukaemia depends on the type of white cell affected.
  • Too few platelets: this is called thrombocytopenia. This may make you bruise or bleed easily. There are various causes.
  • Too many platelets: this is called thrombocytopenia (or thrombocytosis). This is due to disorders which affect cells in the bone marrow, which make platelets.
Other important Blood Tests
  1. Blood Sugar
  2. Liver Function Test
  3. Kidney Function Test
  4. HbA1c Test
  5. Thyroid Test

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