In general, there are four main types of blood tests:

• Hematology tests
• Biochemistry tests
• Microbiology tests
• Serology tests

Hematology tests examine the blood to identify:

• The number and types of blood cells that are present (e.g., red blood cells, white blood cells and platelets)
  
  
• The appearance of the cells, especially their maturity
  
  
• The ability of the blood to form a clot and the speed at which clotting occurs

Biochemistry tests measure the levels of normally occurring chemicals and biochemicals in the blood, both individually and in relation to other chemicals. These measurements are compared to normal ranges and are used to determine whether blood biochemicals are in proper balance. Biochemicals and other substances that may be analyzed include:

• Sodium
• Cholesterol and other fats
• Vitamins and minerals
• Enzymes
• Hormones
• Blood gases
• Certain prescription drugs
• Illegal/recreational drugs
• Alcohol

Biochemical tests not only accurately measure these substances, but they can also be used to indicate how well some organs and systems are functioning. Measuring the amount of certain enzymes in the bloodstream can indirectly reflect the progression of a disease and its effect on the body.

Microbiology tests examine blood for the presence of infectious microscopic organisms such as:

• Bacteria
• Fungi
• Viruses (in most clinical labs a serology test is used)
• Parasites

Microbiology tests include:

• Smears, in which a small amount of blood is placed on a glass slide for examination under a microscope. Sometimes the blood smear is stained with special dyes before examination.
  
  
• Blood cultures, in which a small amount of blood is placed in a nutrient broth, incubated for days or weeks and then examined for growth of disease–causing bacteria.

Serology tests (tests performed on blood serum) can detect the presence of antibodies that are produced by white blood cells to attack microscopic organisms. They are frequently used to detect viral diseases.