A blood test uses a sample of blood from an artery or a vein to detect and measure various factors in the blood. In some cases, it may be just as important to establish that a condition is “normal” as to establish that a condition is “abnormal.” Blood tests serve a number of purposes when used with people who have cardiovascular conditions. These purposes include:

• To help diagnose heart disease or identify risk factors (e.g., abnormal cholesterol or triglyceride levels)
  
  

• To help establish that a heart attack has occurred and measure the extend of damage to the heart by measuring the blood levels of certain cardiac enzymes
  

• To monitor the patient’s progress during treatment

Blood, which is part of the circulatory system, contains a variety of cells suspended in a light-colored fluid known as plasma. Plasma composes more than half of the blood volume. Dissolved in the plasma are many substances, such as electrolytes, nutrients, vitamins, clotting factors, hormones, enzymes and antibodies. 

There are three types of blood cells present in the plasma: red blood cells (carry oxygen to the tissues), white blood cells (fight invading organisms) and platelets (involved in blood clotting). An analysis of various blood components can provide the physician with essential information about its microscopic contents, including:

• Blood cells
  
  
• Nutrients, vitamins and minerals
  
  
• Proteins, sugars and fats
  
  
• Chemicals
  
  
• Hormones
  
  
• Antigens (foreign proteins that stimulate the formation of antibodies)
  
  
• Antibodies (cells that identify foreign proteins for attack by white blood cells or, in the case of autoimmune diseases, mistakenly identify the patient’s own cells as foreign invaders) 
  
  
• Blood gases (e.g., oxygen or carbon dioxide that is dissolved in the blood)
  
  
• enzymes
  
  
• Clotting factors

Because blood carries all of these substances to various parts of the body, examining its contents can help to evaluate the health of major organs and organ systems, including the heart, lungs and respiratory system. For example, blood tests can determine how well the liver, kidney and lungs are functioning and whether there is any infection or inflammation.