Companion diagnostic tests identify specific genetic mutations and biomarkers in individuals who are most likely to benefit from precision medicine treatment, lowering the number of patients who are treated. Companion diagnostics play a pivotal role in the development of targeted medications, and their effectiveness determines the success of this type of therapy to a great extent. Companion diagnostic assays use a single patient as a reference point and will be crucial in the transition to more precise and personalized medication. These are medical gadgets that assist doctors in determining the treatments and dosages to offer patients, based on the patient's individual needs. The companion diagnostic is necessary for the drug's safe and effective use.