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Blood tests: Understanding the basics

Written by Dr Anitha Anchan |Updated : August 6, 2013 11:44 AM IST

Blood plays a vital role in keeping us alive, hale and hearty. It supplies nutrients and oxygen to various organs, muscles and tissues and transports waste products and carbon dioxide away from them. Blood pressure is considered one of the 'vital signs' from a physiological angle and blood tests (however gory they maybe) can provide tons of important information about your health and well-being.

Before understanding why blood tests are done, it is important to know about its different components. An increase/decrease in number/volume, change in their shape or size can indicate abnormalities:

Red Blood Cells (RBCs or erythrocytes) are the most abundant cells in blood. They contain haemoglobin, an iron-containing protein, which is responsible for oxygen transportation around the body.

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White Blood Cells (WBCs or Leukocytes) form part of the body's immune system and help defend the body against infection. They are characterized as granulocytes or agranulocytes. There are three types of granulocytes Neutrophils, Eosinophil and Basophils and two types of agranulocytes Monocytes and Lymphocytes.

Platelets (Thrombocytes) are cells that help the blood to clot (thicken) when you cut yourself

Plasma - constitute 55% of blood fluid and contains proteins, glucose, mineral ions, hormones, carbon dioxide, platelets and blood cells. Plasma proteins also help blood to clot.

Blood tests form the fulcrum of diagnosis in modern medicine. A sample of blood from your vein can help doctors

  • Assess general state of your health
  • Determine whether you have certain diseases and conditions
  • Check the function of organs like liver and kidney
  • Identify risk factors for heart disease
  • Check whether medicines taken are working
  • Grow the infectious organisms (causing an illness) on blood samples and identify them through a microscope
  • Obtain DNA sample for genetic testing

How does a blood test work?

During a blood test, a small amount of blood is taken from your body, usually from a vein in your arm or rarely from an artery in your wrist, using a needle. A finger prick may also be used.

First a tight band (tourniquet) is tied around the upper arm to make the vein prominent. The site of the injection is wiped with an antiseptic wipe. A needle attached to a syringe or to a special blood collecting container is pricked into the vein. The syringe is used to draw out a sample of your blood. When the necessary amount of blood has been taken, the needle is removed. Pressure is applied for a few minutes to site of injection on your skin using a cotton pad to stop the bleeding. A plaster may be put keep it clean and prevent infection. The blood sample will be put into a bottle labelled with your name and sent to a laboratory to be examined under a microscope or tested with chemicals.

Are there any risks involved with blood tests?

Blood tests have few risks. Only a small amount of blood is taken during a blood test, so you will not feel ill from the loss of blood. There may be minor complications which subside shortly after the tests are done.

  • If you are very sensitive to needles and the sight of your own blood, you may feel dizzy when a blood sample is taken. This can be reduced by sitting or lying down while the sample is taken.
  • After a blood test, you may have a small bruised area on your skin where the needle was inserted. Bruises are the result of bleeding under the skin. They can be painful but are harmless and they are most commonly caused by lack of pressure at the site of the needle prick till the bleeding has stopped or a blood vessel being damaged by the needle during its insertion
  • If you develop redness or swelling (inflammation) at the site, you may have an infection. Consult your doctor.
  • There is a risk of getting HIV/AIDS, Hepatitis B or C if a contaminated needle is used during a blood test. Make sure a new syringe is being usedwhen you have your blood taken for tests.

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