Diagnostic agents are substances that are either organic or inorganic used in so many different modalities of investigations to diagnose a disease. Diagnostic agents that are used in imaging are either radioisotopes or radiological contrast agents which can be given through various routes such as oral, injectables, urinary tract and rectal. These substances can be used to assess the function of different organs in the body. Other than that, other diagnostic agents include the culture media or agar used as a base for growing certain bacteria. This method is used for the identification of bacteria that cause certain infections. Examples are blood agar, chocolate agar, anaerobic agar, and acetamide agar. Some agar is specific for a certain type of organism. Dyes and stains are also used as microbiology diagnostic agents for staining microorganisms such as bacteria, fungi, and parasites as well as to stain tissues to be viewed under a microscope.