patient under investigation patient under investigation a patient under investigation (or a person under investigation) refers to a person who had been in close contact with a person with confirmed infection or/and may have been to place where there is an outbreak or superspreading event. this person exhibits the symptoms of the disease and is required to be tested, and undergo a quarantine or isolation while waiting for the laboratory results. it is a term used by health care workers in classifying patients during evaluation and testing in contact tracing in times of infectious disease outbreaks.