A companion diagnostic is a medical device, most commonly an in vitro device, that gives information necessary for the safe and successful use of a corresponding medicine or biological product. The test aids a physician in determining if the benefits of a medicinal product to patients will outweigh any potential serious side effects or hazards. Patients who are most likely to benefit from a certain medicinal product can be identified using companion diagnostics. A companion diagnostic (CDx) is a medical instrument or test that gives information necessary for the safe and successful use of a biological product; in most cases, this would be a highly personalized or focused medication treatment. It allows a doctor to determine if the advantages of a drug or treatment for a patient outweigh any potential serious side effects or hazards. The companion diagnostic is a pharmacogenomics approach. This refers to how a person's complete genome (i.e., their entire genetic pattern, not just individual genes) reacts to the usage of a drug, as well as the drug's total potential influence on that person, including the drug's potential benefits and risks.